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Effect of physical property of supporting media and variable hydraulic loading on hydraulic characteristics of advanced onsite wastewater treatment system a
a
Meena Kumari Sharma & Absar Ahmad Kazmi a
Department of Civil Engineering, Indian Institute of Technology Roorkee, Roorkee, India Accepted author version posted online: 27 Nov 2014.Published online: 22 Dec 2014.
Click for updates To cite this article: Meena Kumari Sharma & Absar Ahmad Kazmi (2014): Effect of physical property of supporting media and variable hydraulic loading on hydraulic characteristics of advanced onsite wastewater treatment system, Environmental Technology, DOI: 10.1080/09593330.2014.992480 To link to this article: http://dx.doi.org/10.1080/09593330.2014.992480
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Environmental Technology, 2014 http://dx.doi.org/10.1080/09593330.2014.992480
Effect of physical property of supporting media and variable hydraulic loading on hydraulic characteristics of advanced onsite wastewater treatment system Meena Kumari Sharma ∗ and Absar Ahmad Kazmi Department of Civil Engineering, Indian Institute of Technology Roorkee, Roorkee, India
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(Received 4 April 2014; accepted 22 November 2014 ) A laboratory-scale study was carried out to investigate the effects of physical properties of the supporting media and variable hydraulic shock loads on the hydraulic characteristics of an advanced onsite wastewater treatment system. The system consisted of two upflow anaerobic reactors (a septic tank and an anaerobic filter) accommodated within a single unit. The study was divided into three phases on the basis of three different supporting media (Aqwise carriers, corrugated ring and baked clay) used in the anaerobic filter. Hydraulic loadings were based on peak flow factor (PFF), varying from one to six, to simulate the actual conditions during onsite wastewater treatment. Hydraulic characteristics of the system were identified on the basis of residence time distribution analyses. The system showed a very good hydraulic efficiency, between 0.86 and 0.93, with the media of highest porosity at the hydraulic loading of PFF ≤ 4. At the higher hydraulic loading of PFF 6 also, an appreciable hydraulic efficiency of 0.74 was observed. The system also showed good chemical oxygen demand and total suspended solids removal efficiency of 80.5% and 82.3%, respectively at the higher hydraulic loading of PFF 6. Plug-flow dispersion model was found to be the most appropriate one to describe the mixing pattern of the system, with different supporting media at variable loading, during the tracer study. Keywords: hydraulic characteristics; hydraulic loading; onsite system; peak flow factor; supporting media
1. Introduction Onsite wastewater treatment systems (OWTS) have been acknowledged as a key component of wastewater treatment infrastructure worldwide. The conventional septic tank (CST) is the most commonly utilized onsite treatment system. However, it has several inherent drawbacks, including low treatment efficiency,[1–5] which may be attributed to its poor hydraulic characteristics and mixing pattern. According to Indian standards, the CST for single household (P.E < 50) is a single anaerobic chamber of either rectangular or cylindrical shape. The hydraulicflow regime in such a system is in the horizontal direction. This hydraulic phenomenon increases the possibility of short circuiting and dead spaces which, in turn, reduce the actual or mean hydraulic retention time (HRT). Moreover, the reduced HRT with the horizontal flow mode significantly diminishes the contact between the incoming substrate and the active biomass accumulated at the bottom of the septic tank, resulting in reduction of the treatment efficiency.[6,7] This inter-relationship between the hydraulic characteristics and treatment efficiency suggests that the treatment efficiency of any bioreactor can be improved by manipulating the hydraulic regime. Another important feature that affects the treatment efficiency is the degree of contact between the incoming
*Corresponding author. Email: [email protected] © 2014 Taylor & Francis
substrate and the active biomass accumulated within the reactor. Several studies have reported that the reactors utilizing a supporting media undergo a greater biomass accumulation per unit volume of the reactor, which increases the efficiency of contact between the substrate and microorganism.[7,8] In addition, the supporting media also affects the HRT and the solid retention time both, which results in an increased treatment efficiency.[9] Therefore, to optimize the treatment efficiency of a reactor, both the parameters (contact with active biomass and the hydraulic characteristics) should be maximized. Therefore, to carry out the present study, a two-stage advanced onsite wastewater treatment system was developed by incorporating certain modifications in the structure and flow-direction of the CST to obtain maximum treatment efficiency. The system consisted of two upflow anaerobic bio-reactors, a modified septic tank followed by an upflow anaerobic filter, accommodated within a single cylindrical unit. The physical properties (specific surface area and porosity) of supporting media also affect the treatment efficiency and the hydraulic characteristics of the reactor.[8,10] The supporting media with relatively higher values of the physical properties (specific surface area and porosity) allow for greater biomass accumulation and a
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M.K. Sharma and A.A. Kazmi
larger distribution of interstitial flow than the media with lower specific surface area and porosity. This results in enhancement of the intermixing as well as the contact between the incoming substrate and the microorganism, subsequently helping in decreasing dead space and increasing mean HRT. Thus, a higher overall treatment efficiency is achieved. However, some studies have revealed that the specific surface area of the supporting media seems to have only a minor effect as compared to pore size and media geometry.[7,10,11] Hence, porosity is considered as the main physical property of the supporting media that affects the performance of the reactor significantly. Normally, the performance of a reactor becomes further complicated by the nature of inflow – variation that an onsite system experiences during a day. These systems generally receive wastewater at a highly variable flow rate that could vary from two to four folds of the average flow.[12,13] Hence, the hydraulic characteristics of the system should be investigated under highly variable hydraulic loading conditions to simulate the actual field state of onsite systems. Generally, the upflow anaerobic reactors are considered to operate as plug-flow reactors (PFRs).[8] However, it has been observed in most of the PFRs that the axial dispersion and exit/entry flow disturbances render the reactor nonideal. Moreover, the supporting media provides interstitial interconnecting pathways for liquid flow, especially in an anaerobic filter, resulting in the prevalence of non-ideal flow.[8,14,15] The non-ideal PFR can be described by axial dispersion models by taking dispersion into consideration.
The extent of dispersion can be accessed from hydraulic characterization. Although, some limited studies have been done on the effect of media,[8,16] there is no information available regarding the effects of supporting media on hydraulic characteristics of an advanced two-stage onsite system under variable hydraulic loading during a day, which is a usual phenomenon in actual field conditions. Thus, the present study investigated the hydraulic characteristics of the system at variable hydraulic loading when the anaerobic-filter part was packed with different supporting media. Furthermore, the treatment efficiency of the system was investigated after selecting the most suitable supporting media by using actual domestic wastewater. The main aim of the study was to develop a low-cost onsite wastewater treatment system which could be highly efficient in terms of hydraulic characteristics and treatment efficiency. 2. Materials and methods 2.1. Reactor configuration The schematic diagram with details of the laboratory-scale set-up is shown in Figure 1. Three laboratory-scale units, each having a working volume of 24 L, were designed and fabricated using transparent acrylic plastic pipes and installed at the campus of Indian Institute of Technology Roorkee, India. Each unit consisted of two chambers and unit dimensions of 450 mm of height and 300 mm of diameter.
Figure 1. Schematic diagram of the laboratory-scale advanced onsite system.
Environmental Technology
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The first chamber of each unit was converted into an upflow-mode reactor by connecting it to a vertical inlet pipe, which was extended down to the bottom of the tank. The second chamber was randomly packed with media and fed with the effluent of the first chamber, which entered it from the bottom. The first chamber was designed to work as an upflow septic tank, while the second one worked as an upflow anaerobic filter.
2.2. Supporting media in anaerobic filter The anaerobic filter chambers of each unit were packed with three different supporting media: Aqwise carriers (ABC5), corrugated ring and baked clay, designated as OWTS-1, OWTS-2 and OWTS-3, respectively (Figure 2). The media had nearly identical particle-size of about 10– 13 mm. All three experimental set-ups were filled with respective media up to the same height. The detailed characteristics of supporting media used in the study are illustrated in Table 1. The media (Aqwise carriers (ABC5) used in OWTS-1 is available commercially and was procured locally. The media used in OWTS-2 was obtained by cutting a corrugated pipe of unit diameter of Ø 11 mm into equal sizes of 10–13 mm length. The media used in OWTS-3 was obtained by crushing the locally available bricks into small pieces and subsequently passing the crushed material through three consecutive sieves of Ø 8, 10 and 12 mm. Finally, the material retained by the sieve of Ø 10 mm was used as the supporting media.
2.3. Feed flow pattern Onsite treatment systems generally receive the wastewater with large variation in quality and quantity both during the course of the day, which depends upon the household activities. Therefore, the lab-scale system was run according to the daily variation in the inflowing wastewater in the actual field conditions. The daily flow variation can be characterized by peak flow factor (PFF), which is the ratio of maximum (Qmax ) to average hourly flow (Qavg ) and can (a)
(b)
3
Table 1. Detailed characteristics of supporting media. Reactor
OWTS-1
OWTS-2
OWTS-3
Supporting media
Aqwise carriers (ABC5) 10–13 93 550
Corrugated ring
Baked clay
10–13 91 103
10–13 46 13
Size (mm) Porosity (%) Cost (Rs. per kg)
be expressed by Equation (1): PFF =
Qmax Qavg
(1)
On the basis of PFF, the flow can be classified into steady and non-steady flow. The system was fed with variable PFF values at a fixed HRT of 24 h. It was assumed that the feed flow with variable PFF (1, 2, 4 and 6) represented all the possible hydraulic shock loading conditions for the household systems.[13] The PFF value of 1 indicated a steady flow, while the PFF values of 2, 4 and 6 fold of average flow indicated a non-steady flow representing the hydraulic shock load. At PFF 1, the system was continuously fed at a constant flow rate of 16.6 mL/min, while at PFF 2, 4 and 6, variable feed flow was supplied during a day. Peak flows were set twice a day for the duration of one hour each in the morning (7.00–8.00 a.m.) and evening (7.00–8.00 p.m.) to simulate the average household wastewater flow pattern.[12] Hourly flow rates before (6.00–7.00 a.m. and 6.00–7.00 p.m.) and after the peak flow periods (8.00–9.00 a.m. and 8.00–9.00 p.m.) were regulated at half of the peak flow for 1 h as per Sarathai et al. [13]. For the remaining 18 h, the system was fed with flow rate of 0.89 and 0.45 times the average flow for PFF 2 and 4, respectively. However, at PFF 6, the system was maintained without any flow of liquid for the remaining 18-h period. In all the possible conditions of feed flow, the total inflow volume was 24 L to maintain the 24-h HRT of the system. (c)
Figure 2. Different media used in the study. (a) Aqwise carriers-ABC5, (b) corrugated ring, (c) baked clay.
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M.K. Sharma and A.A. Kazmi
2.4. Tracer experimental procedure The hydraulic characteristics of the system were determined by residence time distribution (RTD) curves. The RTD curves, the time distribution for particles entering and leaving the system, are obtained from tracer studies and further analysed for mixing pattern.[17] Tracer studies were performed by using step stimulus technique. Lithium chloride (LiCl) was selected as the tracer due to its various favourable features as described by Andersons et al. [18]. Prior to starting the tracer study, the supporting material was allowed to become completely saturated for 24 h with the same concentration of tracer solution. Then the tracer study was carried out after thorough washing of the supporting media to reduce absorption of the tracer into the supporting material during the experiment, which subsequently helped in reducing the chances of any experimental error. A liquid solution containing 100 mg/L of LiCl was fed continuously with the input stream. The tracer concentration C(t) was measured at the outlet point till the concentration of tracer reached a steady state. The response was plotted as a time-record of the tracer concentration leaving the reactor by taking grab samples at intervals of 1 h each for the duration of 6 h. Then the sample collection interval was reduced to 30 min for the successive 12 h. Thereafter, the sample collection interval was further reduced to 15 min till the end of the experiment. The resulting response curve, referred as the C-diagram, provided an accurate representation of the hydraulic regime of the system.[17] Twelve experimental runs of the tracer study were conducted with a clean reactor (without any sludge) to obtain the hydraulic characteristics of the system for investigational purposes (Table 2). The entire tracer study was divided into three phases (A, B and C), each carried out with a different supporting media packed in the anaerobic filter chamber. Furthermore, each phase was divided into four groups on the basis of PFF value. The tracer studies were conducted by operating the system at a fixed HRT of 24 h while feeding it with the simple tap water added with LiCl at variable hydraulic loading at PFF 1, 2, 4 and 6. Table 2.
2.5. Performance with selected media Treatment efficiency of the system was evaluated at variable flow conditions at different PFF values of 1, 2, 4 and 6 when the anaerobic filter was packed with the media selected on the basis of higher hydraulic efficiency. The system was seeded with digested sludge collected from a full-scale UASB reactor located at Saharanpur, India. The sludge containing volatile suspended solids of 21.8 g/L was added into the first chamber up to about 50% of its volume, while the remaining volume of the reactor, including the second chamber, was filled with decanting wastewater of the digested sludge. The reactor was allowed to remain under anaerobic conditions for a week. Thereafter, it was fed with actual domestic wastewater collected daily from the nearby pumping station at Roorkee. The major influent characteristics of collected wastewater in terms of average concentration of chemical oxygen demand (COD) and total suspended solids (TSS) were 421.6 ± 123 and 213.8 ± 68 mg/L, respectively. The treatment efficiency of the system was also identified with baked clay as the supporting media. Baked clay is a construction waste material, locally available in abundance and the cheapest one. Treatment efficiency was evaluated at the four variable hydraulic loading conditions of PFF 1, 2, 4 and 6 after it achieved a steady state for each experimental run. 2.6.
Analytical methods
The influent and effluent wastewater samples were collected and analysed for the main pollutant indices of COD and TSS. These samples were taken for quality assessment in different conditions. The 24-h composite samples were collected for analysis. The concentrations of COD and TSS were measured according to standard methods of examination for water and wastewater.[19] The chloride-ion concentration was measured using a conductivity meter (Model CDC 401, Hach, USA) after suitable calibration with 0.1 N potassium chloride (KCl) solution.
Details of experimental conditions during the tracer study.
Experimental phase
Supporting media
Experimental run
PFF
HRTideal
A
Aqwise carriers (ABC5)
B
Corrugated ring
C
Backed clay
1 2 3 4 5 6 7 8 9 10 11 12
1 2 4 6 1 2 4 6 1 2 4 6
24 24 24 24 24 24 24 24 24 24 24 24
Biomass in reactor
Type of feed
No
Tap water with LiCl as tracer
No
Tap water with LiCl as tracer
No
Tap water with LiCl as tracer
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Environmental Technology 3. Theoretical interpretations 3.1. HRT distribution and dead space calculation Tracer experiment was conducted to investigate the hydraulic characteristics and flow regime based on the analysis of time tracer response curves. For comparison of reactors, it is common to normalize (dimensionless unit) the response curves known as normalized RTD curves.[20] Normalized RTD curves of the tracer study were determined on the basis of dimensionless concentration (Equation (2)) against the dimensionless time (Equation (3)): Ci , (2) Cθi = Co where Cθi is the normalized tracer concentration at dimensionless time i, Ci is the tracer concentration at time i and Co is the initial tracer concentration. Similarly, the normalized time value was calculated using the following equation. θi =
ti , HRTideal
(3)
where θ i is the normalized time at dimensionless time i, ti is the time at i and HRTideal is the theoretical HRT. The normalized RTD curves were characterized for hydraulic regime. To carry out a comprehensive analysis of the hydraulic regime, the RTD functions (E(θ ), F(θ )), mean HRT (θ m ) and distribution variance (σθ2 ) of normalized RTD were calculated by using Equations (4–7).[21] E(θ ) = F(θ ) = θm = θmean σθ2 =
C(θ ) , C(θ )dθ
E(θ )dθ, = θ Cθ dθ ,
(θ − θm ) Cθ dθ = Cθ dθ 2
(4) (5) (6)
θ 2 Cθ dθ − θm2 .
∂C ∂ 2C ∂C =D 2 −u , ∂t ∂x ∂x
(10)
where D is the axial dispersion coefficient, m2 /sec; t is the time; x is the axial distance of the reactor, m; and u is the average fluid velocity in the flow direction, m/sec. A dimensionless form of the Equation (10) can be expressed as Equation (11): 2 ∂ 2 C ∂C D ∂ C ∂C ∂C − − = = D , (11) d ∂θ uL ∂Z 2 ∂Z ∂Z 2 ∂Z where Dd represents dispersion number (Dd = D/uL), C = C(t)/C0 and Z = (ut + x)/L. The dimensionless dispersion number measures the extent of axial dispersion in the treatment unit. A large dispersion number, Dd = ∞, implies a perfectly mixed system, whereas a small dispersion number, Dd = 0, relates to a plug-flow system. Similarly, Dd = 0.02, is defined as intermediate and Dd = 0.2, as a large degree of dispersion. In order to solve Equation (11), it is required to mention the boundary condition of the reactor. For a closed-vessel boundary condition, in which only axial mixing is considered, Equation (12) was used to obtain normalized variance as a function of dispersion number.[21] 2 D D uL −2 1 − e− D , (12) σ2 = 2 uL uL σθ2 . θm2
(7)
where Vd is the dead volume, % Short circuiting () can be expressed as the ratio of the first appearance of tracer (tf ) in the effluent to the theoretical HRT (HRTideal ) as shown in Equation (9): tf . HRTideal
3.2. Dispersion number Hydraulic regime of non-ideal plug flow reactors can be modelled by taking dispersion into consideration. Dispersion coefficient is a general term used to characterize the axial dispersion. Under steady-state condition, the governing equation for mass balance to establish axial dispersion model applying Fick’s law in axial direction is as follows (Equation (10)):
where σ 2 is the dimensionless variance of RTD, σ 2 =
The dead spaces present in a reactor reduce the active volume of the system. The percentage of the dead volume can be calculated by using Equation (8): θm × 100%, (8) Vd = 1 − HRTideal
=
5
(9)
The short-circuiting ratio of 0.3 or less reflects a flow with significant short circuiting.[22]
3.3.
Hydraulic efficiency
The hydraulic efficiency (λ) includes two basic features of the treatment unit: effective volume and flow pattern. It was calculated by using Equation (13): 1 , (13) λ=e 1− N where e is the effective volume and is calculated by subtracting the value of dead space from 1 and N is the flow pattern and is estimated as N = σ12 . Depending on the calculated value of λ, hydraulic efficiency of the system can be classified into three categories: (i) excellent hydraulic efficiency with λ > 0.75; (ii) good hydraulic efficiency with 0.5 < λ ≤ 0.75; and (iii) poor hydraulic efficiency with λ ≤ 0.5.
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M.K. Sharma and A.A. Kazmi the same hydraulic loading, the hydraulic characteristics of the three systems were observed to be more or less similar. Results of analysis of all the RTD curves are summarized in Table 3.
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4. Results and discussion The normalized RTD curves for all experimental runs executed at 24-h HRT are shown in Figure 3(a)–3(d). The tracer recoveries ranged between 90% and 97% in all experimental runs. It was observed that the RTD curves in constant-flow condition at PFF 1 were standardizing to S-shape in each phase of the study performed with different media. However, as the hydraulic loading increased from PFF 1 to 6, the shape of the RTD curves got distorted and accordingly exhibited widely variable hydraulic characteristics. Similar changes were observed in other two systems with different media: OWTS-2 and OWTS-3. However, at
(a)
4.1. HRT distribution in tracer study It was observed from the results that the mean HRT of the system was slightly deflected from the theoretical HRT in all the experimental runs. The system showed a rather similar mean HRT, ranging between 0.81 and 0.95, to theoretical HRT at PFF ≤ 4; while at the PFF 6, it varied within
(b)
(c)
(d)
Figure 3. Normalized RTD curves of the advanced onsite system with different supporting media and variable hydraulic loading at (a) PFF 1, (b) PFF 2, (c) PFF 4, (d) PFF 6. Table 3. Hydraulic characteristics of the advanced onsite wastewater treatment system. System
Run
HRTideal /h
θ m /h
σθ2
Dd
λ
Vd /%
OWTS-1
A-1 A-2 A-4 A-6 B-1 B-2 B-4 B-6 C-1 C-2 C-4 C-6
24 24 24 24 24 24 24 24 24 24 24 24
22.6 21.6 22.9 18.4 21.9 21.7 22.6 18.2 20.7 19.5 21.5 17.9
3.3 4.5 3.4 3.4 0.9 4.4 3.7 1.8 5.4 4.8 4.8 4.2
0.292 0.309 0.311 0.299 0.299 0.302 0.309 0.295 0.208 0.215 0.208 0.295
0.010 0.021 0.011 0.017 0.014 0.020 0.014 0.005 0.034 0.031 0.025 0.028
0.92 0.86 0.93 0.74 0.88 0.87 0.91 0.75 0.81 0.76 0.85 0.70
5.9 9.8 4.7 23.2 8.9 9.4 5.9 24.3 13.6 18.8 10.2 25.5
OWTS-2
OWTS-3
Note: θ m , mean HRT; HRTideal , theoretical HRT; σ θ 2 , variance; , short circuiting; Dd , dispersion number; λ, hydraulic efficiency; Vd , dead space.
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Environmental Technology the range of 0.75–0.77. It was also clear from the results for each system that the reduction in the values of mean HRT was directly related to the decrease in the porosity of the supporting media. It was also found that OWTS-1, having the highest-porosity supporting media, indicated a higher mean HRT at the same value of PFF in comparison to OWTS-2 and OWTS-3. However, the reduction in the mean HRT due to the decrease in the porosity of the media was not quite significant in comparison to the reduction observed due to increasing hydraulic loading, as the mean HRT was highly influenced when the applied PFF was more than 4. Therefore, hydraulic loading at PFF 6 was the major contributor in the reduction of mean HRT in comparison to the physical properties of the media and led to the deviation from the theoretical HRT in each system.
4.2. Dead space and short circuiting Results showed deviation of the actual HRT from the theoretical HRT of 24 h, which indicated the presence of dead space and/or short circuiting within the reactor. The dead space can be categorized into biological dead space and hydraulic dead space. The biological dead space is related to the biomass, whereas the hydraulic dead space is related to the flow rate and hydraulic loading.[17] Thus, the dead space was categorized as hydraulic dead space in all the experimental runs because systems were run without biomass. Results revealed that there was no significant difference in the amount of dead space present within the three systems. It varied between (4.7–9.8%), (5.9–9.4%) and (10.2–18.8%) at hydraulic loading of PFF 1, 2 and 4 respectively among all the three systems. However, at PFF 4, slightly lower dead space was observed than at PFF 2, with all the three supporting media. This might be due to the flow rate of 0.45 times the average flow for eighteen hours at PFF 4, which was 0.89 times the average flow at PFF 2. At PFF 4, there was a lower hydraulic loading for the maximum period of time (18 h) during the designed HRT of 24 h than the hydraulic loading at PFF 2. Such a long period of reduced hydraulic loading contributed significantly to the reduction of dead space at PFF 4. However, at PFF 6, large fraction of dead space (23.2– 25.5%) was found in all the three systems (Table 3). It was obvious from comparison of the data obtained from the tracer study that the dead space in OWTS-3 was more than that in OWTS-1 and OWTS-2, which indicated that the system having the supporting media of higher porosity was able to maintain a larger effective volume. Therefore, it was concluded that the occurrence of dead space was influenced more by the hydraulic loading than by the physical properties of the supporting media. Similarly, the effect of short circuiting was also not found to be very significant and varied within the range of 0.208–0.311 with the increase in the PFF from 1 to 6 in all experimental runs. But at any fixed value of PFF,
7
the results indicated that the short circuiting was higher in OWTS-3 than in OWTS-1 and OWTS-2. The presence of higher short circuiting in OWTS-3 might be due to a relatively low porosity and the irregular shape of the packing media, which led to channelling from the boundary wall and increase in the effect of short circuiting. Overall, the results revealed that the formation of dead space depends upon both the hydraulic loading and the physical properties of the supporting media. Hence, it was concluded that high PFF was the major contributor in the occurrence of short circuiting, higher percentage of the dead space and deviation from the theoretical HRT during the study period.
4.3. Mixing pattern and dispersion number Flow pattern refers to the mixing of materials, which has a considerable bearing on the performance of the reactor and can be described by dispersion numbers Dd . By using the axial dispersion model (Equation (12)), the calculated dispersion numbers of the system were found to vary within the range of 0.005–0.034 in all the experimental runs (Table 3). On the basis of the prescribed criteria, when Dd ≤ 0.2, the present two-stage advanced onsite system worked as an intermediate between plug-flow and completely-mixed flow reactor. For all the experimental runs of OWTS-1 and OWTS-2, the mixing pattern approached to plug flow with intermediate dispersion when the Dd ≤ 0.02. Generally, the performance of the PFR is higher than the other reactors in which dispersion number (Dd ≥ 0.2) approached to completely mixed flow reactors.[23] Therefore, the systems OWTS-1 and OWTS-2 had a greater capacity of pollutant conversion than OWTS-3 at the same hydraulic loading.
4.4. System performance 4.4.1. Hydraulic efficiency The hydraulic efficiency speculates two basic features: the ability for uniform distribution of inflow and the amount of mixing.[13,24] As mentioned above, no significant difference was observed in the dead space and mixing pattern, which indicates an even distribution of inflow in all three systems at the hydraulic loading of PFF ≤ 4. Thus, the hydraulic efficiency of the system was not significantly different and ranged from 0.76 to 0.93 at PFF 1, 2 and 4. However, it tended to decline with the decreasing porosity of the supporting media. At the hydraulic loading PFF 6, satisfactory hydraulic efficiency was observed with the values of 0.74, 0.75 and 0.70 for OWTS-1, OWTS-2 and OWTS-3, respectively. On the basis of the criteria discussed in Section 2.3.1, OWTS-1 and OWTS-2 represented a good hydraulic efficiency, whereas OWTS-3 fell into the category of good to satisfactory, in all the phases of variable hydraulic loading. The system OWTS-3 indicated a
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M.K. Sharma and A.A. Kazmi
140
COD
TSS 71.8%
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Effluent concentration (mg/L)
120 100 80.5%
80
81.2% 84.3%
85.1%
87.6%
60
78.1%
90.3%
91.2%
40
82.3% 86.7%
87.8% 90.2%
91.8%
92.1%
88.3%
20 0 A(1)
A(2)
A(4)
A(6)
C(1)
C(2)
C(4)
C(6)
Experimental run with different media at variable PFF
Figure 4. Effluent concentrations with respective removal efficiency for COD and TSS.
lower hydraulic efficiency than the other two (OWTS-1 and OWTS-2). This might be due to the lower porosity (46%) of the supporting media present in OWTS-3 than the porosity of 93% and 91% of the supporting media present in OWTS-1 and OWTS-2, respectively. Thus, the results indicated that the porosity of the media and the hydraulic loading both affected the hydraulic efficiency. On the basis of comparative analysis of the results of hydraulic characteristics of OWTS-1, OWTS-2 and OWTS-3, Aqwise carrier (ABC5) was finally selected as the packing material.
4.4.2. Treatment efficiency The performance efficiency of OWTS-1 in terms of removal of COD and TSS was found to be 90.3% and 92.1%, respectively at the steady-flow condition at 24-h HRT. It was considered as the baseline performance. At the non-steady flow condition, the removal efficiency was slightly affected as the loading increased from PFF 1 to 6. It was observed that the performance of the system significantly decreased at the hydraulic loading of PFF 6 in all the phases of the study. The overall COD and TSS removal efficiency achieved at this condition of high PFF was more than 80.5% and 82.3%, respectively. The performance of the system with baked clay media (OWTS-3) was also evaluated to develop a low-cost onsite system. Furthermore, the performances of the systems OWTS-1 and OWTS-3 were compared for relative efficiency at variable hydraulic loading of PFF as illustrated in Figure 4. For OWTS-3, the pollutant removal efficiency
in terms of COD and TSS was observed to be 71.8% and 78.1%, respectively, for the higher loading at PFF 6. When a comparison of the treatment efficiency of OWTS-1 and OWTS-3 was done, it was noted that OWTS-3 achieved approximately 6%, 8%, 7% and 11% lower COD removal efficiency at PFF 1, 2 4 and 6, respectively, than OWTS-1. Thus, the system OWTS-3 exhibited satisfactory performance with a slightly lower efficiency than OWTS-1. This might be attributed to the lower values of the physical properties of the supporting media present in OWTS-3 than the supporting media present in OWTS-1. Both OWTS1 and OWTS-3 indicated a good stability as the standard deviation of effluent COD and TSS concentration did not exceed 11.0 and 8.7 mg/L, respectively, in all the possible conditions of hydraulic loading. High treatment efficiency with a good resilience even at a high PFF was attributable to the new configuration of the present onsite system, which also enhanced the hydraulic characteristics. In addition, the appropriate physical properties of the supporting media further increased the pollutant removal efficiency.
5. Conclusions The present system showed a considerable potential to be utilized as an onsite domestic wastewater treatment system. The system showed a good to satisfactory hydraulic efficiency in all the phases of hydraulic loading conditions at PFF 1, 2, 4 and 6 with the three different supporting media (Aqwise carrier media, corrugated ring media and baked clay media) having different physical properties
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Environmental Technology (specific surface area and porosity). It was observed that there was greater impact of hydraulic loading at high PFF on the hydraulic characteristics than the physical properties of the supporting media. The treatment efficiency of the system having Aqwise carrier media (OWTS-1) was slightly higher than the system having baked clay media (OWTS-3), which was attributable to the higher porosity of Aqwise carrier media. The higher porosity of Aqwise carrier media provided greater accumulation of biomass and maximum distribution of interstitial flow, which resulted in enhancement of intermixing as well as contact between incoming substrate and microorganism. However, the performances of both the systems were highly appreciable with a good resilience during the non-steady flow conditions and the credit goes to the unique configuration of the present system, which minimized the effect of variable hydraulic shock loadings. Hence, it is suggested here that backed clay can be utilized as a better option of supporting media than Aqwise carrier media in the development of a low-cost advanced onsite domestic wastewater treatment system, especially for the economically backward rural and peri-urban areas of the developing countries. Disclosure statement No potential conflict of interest was reported by the author(s).
Funding This work is financially supported by the Ministry of Drinking Water Supply and Sanitation, Government of India, New Delhi, India through project number MRD-553-CED.
References [1] Chen Z, Wen Q, Guan H, Bakke R, Ren N. Anaerobic treatment of domestic sewage in modified septic tanks at low temperature. Environ Technol. 2014;35(17):2123–2131. [2] Sharma MK, Khursheed A, Kazmi AA. Modified septic tank-anaerobic filter unit as a two-stage onsite domestic wastewater treatment system. Environ Technol. 2014;35(17):2183–2193. [3] Nasr FA, Mikhaeil B. Treatment of domestic wastewater using conventional and baffled septic tanks. Environ Technol. 2013;34(16):2337–2343. [4] Cullimore DR, Viraraghavan T. Microbiological aspects of anaerobic filter treatment of septic tank effluent at low temperatures. Environ Technol. 2008;15:165–173. [5] Van Haandel A, Kato MT, Cavalcanti PFF, Florencio L. Anaerobic reactor design concepts for the treatment of domestic wastewater. Rev Environ Sci Biotechnol. 2006;5:21–38.
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[6] Mansouri Y, Zinatizadeh AA, Mohammadi P, Irandoust M, Akhbari A, Davoodi R. Hydraulic characteristics analysis of an anaerobic rotatory biological contactor (AnRBC) using tracer experiments and response surface methodology (RSM). Korean J Chem Eng. 2012;29:891–902. [7] Young JC. Factors affecting the design and performance of upflow anaerobic filters. Water Sci Technol. 1991;24:133– 155. [8] Tay JH, Show KY. Media-induced hydraulic behaviour and performance of upflow biofilters. J Environ Eng. 1998;124:720–729. [9] Spychała M, Bła¨zejewski R, Nawrot T. Performance of innovative textile biofilters for domestic wastewater treatment. Environ Technol. 2013;34(2):157–163. [10] Young JC, Dahab MF. Effect of media design on the performance of fixed-bed anaerobic filters. Water Sci Technol. 1983;15:369–383. [11] Gourari S, Achkari-Begdouri A. Use of baked clay media as biomass supports for anaerobic filters. Appl Clay Sci. 1997;12:365–375. [12] Butler D, Friedler E, Gatt K. Characterising the quantity and quality of domestic wastewater inflows. Water Sci Technol. 1995;31:13–24. [13] Sarathai Y, Koottatep T, Morel A. Hydraulic characteristics of an anaerobic baffled reactor as onsite wastewater treatment system. J Environ Sci. 2010;22:1319–1326. [14] Tembhurkar AR, Mhaisalkar VA. Study of hydrodynamic behavior of a laboratory scale upflow anaerobic fixed film fixed bed reactor. J Environ Sci Eng. 2006;48:75. [15] Young HW, Young JC. Hydraulic characteristics of upflow anaerobic filters. J Environ Eng. 1988;114:621–638. [16] Sharvelle S, McLamore E, Banks MK. Hydrodynamic characteristics in biotrickling filters as affected by packing material and hydraulic loading rate. J Environ Eng. 2008;134:346–352. [17] Grobicki A, Stuckey DC. Hydrodynamic characteristics of the anaerobic baffled reactor. Water Res. 1992;26:371–378. [18] Anderson GK, Campos CMM, Chernicharo CAL, Smith LC. Evaluation of the inhibitory effects of lithium when used as a tracer for anaerobic digesters. Water Res. 1991;25:755–760. [19] APHA/AWWA/WEF. Standard methods for the examination of water and wastewater. 20th ed. Washington, DC: American Public Health Association/American Water Works Association/Water Environment Federation; 1998. [20] Morgan-Sagastume JM, Jimenez B, Noyola A. Tracer studies in a laboratory and pilot scale UASB reactor. Environ Technol. 1997;18:817–825. [21] Levenspiel O. Chemical Reaction Engineering. 3rd ed. New York: John Wiley and Sons; 1999. [22] Thackston EL, Shields Jr. FD, Schroeder PR. Residence time distributions of shallow basins. J Environ Eng. 1987;113:1319–1332. [23] Tomlinson EJ, Chambers B. The effect of longitudinal mixing on the settleability of activated sludge. Technical Report TR 122, Stevenage, England; 1979. [24] Persson J, Somes NLG, Wong THF. Hydraulics efficiency of constructed wetlands and ponds. Water Sci Technol. 1999;40:291–300.
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Effect of physical property of supporting media and variable hydraulic loading on hydraulic characteristics of advanced onsite wastewater treatment system a
a
Meena Kumari Sharma & Absar Ahmad Kazmi a
Department of Civil Engineering, Indian Institute of Technology Roorkee, Roorkee, India Accepted author version posted online: 27 Nov 2014.Published online: 22 Dec 2014.
Click for updates To cite this article: Meena Kumari Sharma & Absar Ahmad Kazmi (2014): Effect of physical property of supporting media and variable hydraulic loading on hydraulic characteristics of advanced onsite wastewater treatment system, Environmental Technology, DOI: 10.1080/09593330.2014.992480 To link to this article: http://dx.doi.org/10.1080/09593330.2014.992480
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Environmental Technology, 2014 http://dx.doi.org/10.1080/09593330.2014.992480
Effect of physical property of supporting media and variable hydraulic loading on hydraulic characteristics of advanced onsite wastewater treatment system Meena Kumari Sharma ∗ and Absar Ahmad Kazmi Department of Civil Engineering, Indian Institute of Technology Roorkee, Roorkee, India
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(Received 4 April 2014; accepted 22 November 2014 ) A laboratory-scale study was carried out to investigate the effects of physical properties of the supporting media and variable hydraulic shock loads on the hydraulic characteristics of an advanced onsite wastewater treatment system. The system consisted of two upflow anaerobic reactors (a septic tank and an anaerobic filter) accommodated within a single unit. The study was divided into three phases on the basis of three different supporting media (Aqwise carriers, corrugated ring and baked clay) used in the anaerobic filter. Hydraulic loadings were based on peak flow factor (PFF), varying from one to six, to simulate the actual conditions during onsite wastewater treatment. Hydraulic characteristics of the system were identified on the basis of residence time distribution analyses. The system showed a very good hydraulic efficiency, between 0.86 and 0.93, with the media of highest porosity at the hydraulic loading of PFF ≤ 4. At the higher hydraulic loading of PFF 6 also, an appreciable hydraulic efficiency of 0.74 was observed. The system also showed good chemical oxygen demand and total suspended solids removal efficiency of 80.5% and 82.3%, respectively at the higher hydraulic loading of PFF 6. Plug-flow dispersion model was found to be the most appropriate one to describe the mixing pattern of the system, with different supporting media at variable loading, during the tracer study. Keywords: hydraulic characteristics; hydraulic loading; onsite system; peak flow factor; supporting media
1. Introduction Onsite wastewater treatment systems (OWTS) have been acknowledged as a key component of wastewater treatment infrastructure worldwide. The conventional septic tank (CST) is the most commonly utilized onsite treatment system. However, it has several inherent drawbacks, including low treatment efficiency,[1–5] which may be attributed to its poor hydraulic characteristics and mixing pattern. According to Indian standards, the CST for single household (P.E < 50) is a single anaerobic chamber of either rectangular or cylindrical shape. The hydraulicflow regime in such a system is in the horizontal direction. This hydraulic phenomenon increases the possibility of short circuiting and dead spaces which, in turn, reduce the actual or mean hydraulic retention time (HRT). Moreover, the reduced HRT with the horizontal flow mode significantly diminishes the contact between the incoming substrate and the active biomass accumulated at the bottom of the septic tank, resulting in reduction of the treatment efficiency.[6,7] This inter-relationship between the hydraulic characteristics and treatment efficiency suggests that the treatment efficiency of any bioreactor can be improved by manipulating the hydraulic regime. Another important feature that affects the treatment efficiency is the degree of contact between the incoming
*Corresponding author. Email: [email protected] © 2014 Taylor & Francis
substrate and the active biomass accumulated within the reactor. Several studies have reported that the reactors utilizing a supporting media undergo a greater biomass accumulation per unit volume of the reactor, which increases the efficiency of contact between the substrate and microorganism.[7,8] In addition, the supporting media also affects the HRT and the solid retention time both, which results in an increased treatment efficiency.[9] Therefore, to optimize the treatment efficiency of a reactor, both the parameters (contact with active biomass and the hydraulic characteristics) should be maximized. Therefore, to carry out the present study, a two-stage advanced onsite wastewater treatment system was developed by incorporating certain modifications in the structure and flow-direction of the CST to obtain maximum treatment efficiency. The system consisted of two upflow anaerobic bio-reactors, a modified septic tank followed by an upflow anaerobic filter, accommodated within a single cylindrical unit. The physical properties (specific surface area and porosity) of supporting media also affect the treatment efficiency and the hydraulic characteristics of the reactor.[8,10] The supporting media with relatively higher values of the physical properties (specific surface area and porosity) allow for greater biomass accumulation and a
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M.K. Sharma and A.A. Kazmi
larger distribution of interstitial flow than the media with lower specific surface area and porosity. This results in enhancement of the intermixing as well as the contact between the incoming substrate and the microorganism, subsequently helping in decreasing dead space and increasing mean HRT. Thus, a higher overall treatment efficiency is achieved. However, some studies have revealed that the specific surface area of the supporting media seems to have only a minor effect as compared to pore size and media geometry.[7,10,11] Hence, porosity is considered as the main physical property of the supporting media that affects the performance of the reactor significantly. Normally, the performance of a reactor becomes further complicated by the nature of inflow – variation that an onsite system experiences during a day. These systems generally receive wastewater at a highly variable flow rate that could vary from two to four folds of the average flow.[12,13] Hence, the hydraulic characteristics of the system should be investigated under highly variable hydraulic loading conditions to simulate the actual field state of onsite systems. Generally, the upflow anaerobic reactors are considered to operate as plug-flow reactors (PFRs).[8] However, it has been observed in most of the PFRs that the axial dispersion and exit/entry flow disturbances render the reactor nonideal. Moreover, the supporting media provides interstitial interconnecting pathways for liquid flow, especially in an anaerobic filter, resulting in the prevalence of non-ideal flow.[8,14,15] The non-ideal PFR can be described by axial dispersion models by taking dispersion into consideration.
The extent of dispersion can be accessed from hydraulic characterization. Although, some limited studies have been done on the effect of media,[8,16] there is no information available regarding the effects of supporting media on hydraulic characteristics of an advanced two-stage onsite system under variable hydraulic loading during a day, which is a usual phenomenon in actual field conditions. Thus, the present study investigated the hydraulic characteristics of the system at variable hydraulic loading when the anaerobic-filter part was packed with different supporting media. Furthermore, the treatment efficiency of the system was investigated after selecting the most suitable supporting media by using actual domestic wastewater. The main aim of the study was to develop a low-cost onsite wastewater treatment system which could be highly efficient in terms of hydraulic characteristics and treatment efficiency. 2. Materials and methods 2.1. Reactor configuration The schematic diagram with details of the laboratory-scale set-up is shown in Figure 1. Three laboratory-scale units, each having a working volume of 24 L, were designed and fabricated using transparent acrylic plastic pipes and installed at the campus of Indian Institute of Technology Roorkee, India. Each unit consisted of two chambers and unit dimensions of 450 mm of height and 300 mm of diameter.
Figure 1. Schematic diagram of the laboratory-scale advanced onsite system.
Environmental Technology
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The first chamber of each unit was converted into an upflow-mode reactor by connecting it to a vertical inlet pipe, which was extended down to the bottom of the tank. The second chamber was randomly packed with media and fed with the effluent of the first chamber, which entered it from the bottom. The first chamber was designed to work as an upflow septic tank, while the second one worked as an upflow anaerobic filter.
2.2. Supporting media in anaerobic filter The anaerobic filter chambers of each unit were packed with three different supporting media: Aqwise carriers (ABC5), corrugated ring and baked clay, designated as OWTS-1, OWTS-2 and OWTS-3, respectively (Figure 2). The media had nearly identical particle-size of about 10– 13 mm. All three experimental set-ups were filled with respective media up to the same height. The detailed characteristics of supporting media used in the study are illustrated in Table 1. The media (Aqwise carriers (ABC5) used in OWTS-1 is available commercially and was procured locally. The media used in OWTS-2 was obtained by cutting a corrugated pipe of unit diameter of Ø 11 mm into equal sizes of 10–13 mm length. The media used in OWTS-3 was obtained by crushing the locally available bricks into small pieces and subsequently passing the crushed material through three consecutive sieves of Ø 8, 10 and 12 mm. Finally, the material retained by the sieve of Ø 10 mm was used as the supporting media.
2.3. Feed flow pattern Onsite treatment systems generally receive the wastewater with large variation in quality and quantity both during the course of the day, which depends upon the household activities. Therefore, the lab-scale system was run according to the daily variation in the inflowing wastewater in the actual field conditions. The daily flow variation can be characterized by peak flow factor (PFF), which is the ratio of maximum (Qmax ) to average hourly flow (Qavg ) and can (a)
(b)
3
Table 1. Detailed characteristics of supporting media. Reactor
OWTS-1
OWTS-2
OWTS-3
Supporting media
Aqwise carriers (ABC5) 10–13 93 550
Corrugated ring
Baked clay
10–13 91 103
10–13 46 13
Size (mm) Porosity (%) Cost (Rs. per kg)
be expressed by Equation (1): PFF =
Qmax Qavg
(1)
On the basis of PFF, the flow can be classified into steady and non-steady flow. The system was fed with variable PFF values at a fixed HRT of 24 h. It was assumed that the feed flow with variable PFF (1, 2, 4 and 6) represented all the possible hydraulic shock loading conditions for the household systems.[13] The PFF value of 1 indicated a steady flow, while the PFF values of 2, 4 and 6 fold of average flow indicated a non-steady flow representing the hydraulic shock load. At PFF 1, the system was continuously fed at a constant flow rate of 16.6 mL/min, while at PFF 2, 4 and 6, variable feed flow was supplied during a day. Peak flows were set twice a day for the duration of one hour each in the morning (7.00–8.00 a.m.) and evening (7.00–8.00 p.m.) to simulate the average household wastewater flow pattern.[12] Hourly flow rates before (6.00–7.00 a.m. and 6.00–7.00 p.m.) and after the peak flow periods (8.00–9.00 a.m. and 8.00–9.00 p.m.) were regulated at half of the peak flow for 1 h as per Sarathai et al. [13]. For the remaining 18 h, the system was fed with flow rate of 0.89 and 0.45 times the average flow for PFF 2 and 4, respectively. However, at PFF 6, the system was maintained without any flow of liquid for the remaining 18-h period. In all the possible conditions of feed flow, the total inflow volume was 24 L to maintain the 24-h HRT of the system. (c)
Figure 2. Different media used in the study. (a) Aqwise carriers-ABC5, (b) corrugated ring, (c) baked clay.
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M.K. Sharma and A.A. Kazmi
2.4. Tracer experimental procedure The hydraulic characteristics of the system were determined by residence time distribution (RTD) curves. The RTD curves, the time distribution for particles entering and leaving the system, are obtained from tracer studies and further analysed for mixing pattern.[17] Tracer studies were performed by using step stimulus technique. Lithium chloride (LiCl) was selected as the tracer due to its various favourable features as described by Andersons et al. [18]. Prior to starting the tracer study, the supporting material was allowed to become completely saturated for 24 h with the same concentration of tracer solution. Then the tracer study was carried out after thorough washing of the supporting media to reduce absorption of the tracer into the supporting material during the experiment, which subsequently helped in reducing the chances of any experimental error. A liquid solution containing 100 mg/L of LiCl was fed continuously with the input stream. The tracer concentration C(t) was measured at the outlet point till the concentration of tracer reached a steady state. The response was plotted as a time-record of the tracer concentration leaving the reactor by taking grab samples at intervals of 1 h each for the duration of 6 h. Then the sample collection interval was reduced to 30 min for the successive 12 h. Thereafter, the sample collection interval was further reduced to 15 min till the end of the experiment. The resulting response curve, referred as the C-diagram, provided an accurate representation of the hydraulic regime of the system.[17] Twelve experimental runs of the tracer study were conducted with a clean reactor (without any sludge) to obtain the hydraulic characteristics of the system for investigational purposes (Table 2). The entire tracer study was divided into three phases (A, B and C), each carried out with a different supporting media packed in the anaerobic filter chamber. Furthermore, each phase was divided into four groups on the basis of PFF value. The tracer studies were conducted by operating the system at a fixed HRT of 24 h while feeding it with the simple tap water added with LiCl at variable hydraulic loading at PFF 1, 2, 4 and 6. Table 2.
2.5. Performance with selected media Treatment efficiency of the system was evaluated at variable flow conditions at different PFF values of 1, 2, 4 and 6 when the anaerobic filter was packed with the media selected on the basis of higher hydraulic efficiency. The system was seeded with digested sludge collected from a full-scale UASB reactor located at Saharanpur, India. The sludge containing volatile suspended solids of 21.8 g/L was added into the first chamber up to about 50% of its volume, while the remaining volume of the reactor, including the second chamber, was filled with decanting wastewater of the digested sludge. The reactor was allowed to remain under anaerobic conditions for a week. Thereafter, it was fed with actual domestic wastewater collected daily from the nearby pumping station at Roorkee. The major influent characteristics of collected wastewater in terms of average concentration of chemical oxygen demand (COD) and total suspended solids (TSS) were 421.6 ± 123 and 213.8 ± 68 mg/L, respectively. The treatment efficiency of the system was also identified with baked clay as the supporting media. Baked clay is a construction waste material, locally available in abundance and the cheapest one. Treatment efficiency was evaluated at the four variable hydraulic loading conditions of PFF 1, 2, 4 and 6 after it achieved a steady state for each experimental run. 2.6.
Analytical methods
The influent and effluent wastewater samples were collected and analysed for the main pollutant indices of COD and TSS. These samples were taken for quality assessment in different conditions. The 24-h composite samples were collected for analysis. The concentrations of COD and TSS were measured according to standard methods of examination for water and wastewater.[19] The chloride-ion concentration was measured using a conductivity meter (Model CDC 401, Hach, USA) after suitable calibration with 0.1 N potassium chloride (KCl) solution.
Details of experimental conditions during the tracer study.
Experimental phase
Supporting media
Experimental run
PFF
HRTideal
A
Aqwise carriers (ABC5)
B
Corrugated ring
C
Backed clay
1 2 3 4 5 6 7 8 9 10 11 12
1 2 4 6 1 2 4 6 1 2 4 6
24 24 24 24 24 24 24 24 24 24 24 24
Biomass in reactor
Type of feed
No
Tap water with LiCl as tracer
No
Tap water with LiCl as tracer
No
Tap water with LiCl as tracer
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Environmental Technology 3. Theoretical interpretations 3.1. HRT distribution and dead space calculation Tracer experiment was conducted to investigate the hydraulic characteristics and flow regime based on the analysis of time tracer response curves. For comparison of reactors, it is common to normalize (dimensionless unit) the response curves known as normalized RTD curves.[20] Normalized RTD curves of the tracer study were determined on the basis of dimensionless concentration (Equation (2)) against the dimensionless time (Equation (3)): Ci , (2) Cθi = Co where Cθi is the normalized tracer concentration at dimensionless time i, Ci is the tracer concentration at time i and Co is the initial tracer concentration. Similarly, the normalized time value was calculated using the following equation. θi =
ti , HRTideal
(3)
where θ i is the normalized time at dimensionless time i, ti is the time at i and HRTideal is the theoretical HRT. The normalized RTD curves were characterized for hydraulic regime. To carry out a comprehensive analysis of the hydraulic regime, the RTD functions (E(θ ), F(θ )), mean HRT (θ m ) and distribution variance (σθ2 ) of normalized RTD were calculated by using Equations (4–7).[21] E(θ ) = F(θ ) = θm = θmean σθ2 =
C(θ ) , C(θ )dθ
E(θ )dθ, = θ Cθ dθ ,
(θ − θm ) Cθ dθ = Cθ dθ 2
(4) (5) (6)
θ 2 Cθ dθ − θm2 .
∂C ∂ 2C ∂C =D 2 −u , ∂t ∂x ∂x
(10)
where D is the axial dispersion coefficient, m2 /sec; t is the time; x is the axial distance of the reactor, m; and u is the average fluid velocity in the flow direction, m/sec. A dimensionless form of the Equation (10) can be expressed as Equation (11): 2 ∂ 2 C ∂C D ∂ C ∂C ∂C − − = = D , (11) d ∂θ uL ∂Z 2 ∂Z ∂Z 2 ∂Z where Dd represents dispersion number (Dd = D/uL), C = C(t)/C0 and Z = (ut + x)/L. The dimensionless dispersion number measures the extent of axial dispersion in the treatment unit. A large dispersion number, Dd = ∞, implies a perfectly mixed system, whereas a small dispersion number, Dd = 0, relates to a plug-flow system. Similarly, Dd = 0.02, is defined as intermediate and Dd = 0.2, as a large degree of dispersion. In order to solve Equation (11), it is required to mention the boundary condition of the reactor. For a closed-vessel boundary condition, in which only axial mixing is considered, Equation (12) was used to obtain normalized variance as a function of dispersion number.[21] 2 D D uL −2 1 − e− D , (12) σ2 = 2 uL uL σθ2 . θm2
(7)
where Vd is the dead volume, % Short circuiting () can be expressed as the ratio of the first appearance of tracer (tf ) in the effluent to the theoretical HRT (HRTideal ) as shown in Equation (9): tf . HRTideal
3.2. Dispersion number Hydraulic regime of non-ideal plug flow reactors can be modelled by taking dispersion into consideration. Dispersion coefficient is a general term used to characterize the axial dispersion. Under steady-state condition, the governing equation for mass balance to establish axial dispersion model applying Fick’s law in axial direction is as follows (Equation (10)):
where σ 2 is the dimensionless variance of RTD, σ 2 =
The dead spaces present in a reactor reduce the active volume of the system. The percentage of the dead volume can be calculated by using Equation (8): θm × 100%, (8) Vd = 1 − HRTideal
=
5
(9)
The short-circuiting ratio of 0.3 or less reflects a flow with significant short circuiting.[22]
3.3.
Hydraulic efficiency
The hydraulic efficiency (λ) includes two basic features of the treatment unit: effective volume and flow pattern. It was calculated by using Equation (13): 1 , (13) λ=e 1− N where e is the effective volume and is calculated by subtracting the value of dead space from 1 and N is the flow pattern and is estimated as N = σ12 . Depending on the calculated value of λ, hydraulic efficiency of the system can be classified into three categories: (i) excellent hydraulic efficiency with λ > 0.75; (ii) good hydraulic efficiency with 0.5 < λ ≤ 0.75; and (iii) poor hydraulic efficiency with λ ≤ 0.5.
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M.K. Sharma and A.A. Kazmi the same hydraulic loading, the hydraulic characteristics of the three systems were observed to be more or less similar. Results of analysis of all the RTD curves are summarized in Table 3.
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4. Results and discussion The normalized RTD curves for all experimental runs executed at 24-h HRT are shown in Figure 3(a)–3(d). The tracer recoveries ranged between 90% and 97% in all experimental runs. It was observed that the RTD curves in constant-flow condition at PFF 1 were standardizing to S-shape in each phase of the study performed with different media. However, as the hydraulic loading increased from PFF 1 to 6, the shape of the RTD curves got distorted and accordingly exhibited widely variable hydraulic characteristics. Similar changes were observed in other two systems with different media: OWTS-2 and OWTS-3. However, at
(a)
4.1. HRT distribution in tracer study It was observed from the results that the mean HRT of the system was slightly deflected from the theoretical HRT in all the experimental runs. The system showed a rather similar mean HRT, ranging between 0.81 and 0.95, to theoretical HRT at PFF ≤ 4; while at the PFF 6, it varied within
(b)
(c)
(d)
Figure 3. Normalized RTD curves of the advanced onsite system with different supporting media and variable hydraulic loading at (a) PFF 1, (b) PFF 2, (c) PFF 4, (d) PFF 6. Table 3. Hydraulic characteristics of the advanced onsite wastewater treatment system. System
Run
HRTideal /h
θ m /h
σθ2
Dd
λ
Vd /%
OWTS-1
A-1 A-2 A-4 A-6 B-1 B-2 B-4 B-6 C-1 C-2 C-4 C-6
24 24 24 24 24 24 24 24 24 24 24 24
22.6 21.6 22.9 18.4 21.9 21.7 22.6 18.2 20.7 19.5 21.5 17.9
3.3 4.5 3.4 3.4 0.9 4.4 3.7 1.8 5.4 4.8 4.8 4.2
0.292 0.309 0.311 0.299 0.299 0.302 0.309 0.295 0.208 0.215 0.208 0.295
0.010 0.021 0.011 0.017 0.014 0.020 0.014 0.005 0.034 0.031 0.025 0.028
0.92 0.86 0.93 0.74 0.88 0.87 0.91 0.75 0.81 0.76 0.85 0.70
5.9 9.8 4.7 23.2 8.9 9.4 5.9 24.3 13.6 18.8 10.2 25.5
OWTS-2
OWTS-3
Note: θ m , mean HRT; HRTideal , theoretical HRT; σ θ 2 , variance; , short circuiting; Dd , dispersion number; λ, hydraulic efficiency; Vd , dead space.
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Environmental Technology the range of 0.75–0.77. It was also clear from the results for each system that the reduction in the values of mean HRT was directly related to the decrease in the porosity of the supporting media. It was also found that OWTS-1, having the highest-porosity supporting media, indicated a higher mean HRT at the same value of PFF in comparison to OWTS-2 and OWTS-3. However, the reduction in the mean HRT due to the decrease in the porosity of the media was not quite significant in comparison to the reduction observed due to increasing hydraulic loading, as the mean HRT was highly influenced when the applied PFF was more than 4. Therefore, hydraulic loading at PFF 6 was the major contributor in the reduction of mean HRT in comparison to the physical properties of the media and led to the deviation from the theoretical HRT in each system.
4.2. Dead space and short circuiting Results showed deviation of the actual HRT from the theoretical HRT of 24 h, which indicated the presence of dead space and/or short circuiting within the reactor. The dead space can be categorized into biological dead space and hydraulic dead space. The biological dead space is related to the biomass, whereas the hydraulic dead space is related to the flow rate and hydraulic loading.[17] Thus, the dead space was categorized as hydraulic dead space in all the experimental runs because systems were run without biomass. Results revealed that there was no significant difference in the amount of dead space present within the three systems. It varied between (4.7–9.8%), (5.9–9.4%) and (10.2–18.8%) at hydraulic loading of PFF 1, 2 and 4 respectively among all the three systems. However, at PFF 4, slightly lower dead space was observed than at PFF 2, with all the three supporting media. This might be due to the flow rate of 0.45 times the average flow for eighteen hours at PFF 4, which was 0.89 times the average flow at PFF 2. At PFF 4, there was a lower hydraulic loading for the maximum period of time (18 h) during the designed HRT of 24 h than the hydraulic loading at PFF 2. Such a long period of reduced hydraulic loading contributed significantly to the reduction of dead space at PFF 4. However, at PFF 6, large fraction of dead space (23.2– 25.5%) was found in all the three systems (Table 3). It was obvious from comparison of the data obtained from the tracer study that the dead space in OWTS-3 was more than that in OWTS-1 and OWTS-2, which indicated that the system having the supporting media of higher porosity was able to maintain a larger effective volume. Therefore, it was concluded that the occurrence of dead space was influenced more by the hydraulic loading than by the physical properties of the supporting media. Similarly, the effect of short circuiting was also not found to be very significant and varied within the range of 0.208–0.311 with the increase in the PFF from 1 to 6 in all experimental runs. But at any fixed value of PFF,
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the results indicated that the short circuiting was higher in OWTS-3 than in OWTS-1 and OWTS-2. The presence of higher short circuiting in OWTS-3 might be due to a relatively low porosity and the irregular shape of the packing media, which led to channelling from the boundary wall and increase in the effect of short circuiting. Overall, the results revealed that the formation of dead space depends upon both the hydraulic loading and the physical properties of the supporting media. Hence, it was concluded that high PFF was the major contributor in the occurrence of short circuiting, higher percentage of the dead space and deviation from the theoretical HRT during the study period.
4.3. Mixing pattern and dispersion number Flow pattern refers to the mixing of materials, which has a considerable bearing on the performance of the reactor and can be described by dispersion numbers Dd . By using the axial dispersion model (Equation (12)), the calculated dispersion numbers of the system were found to vary within the range of 0.005–0.034 in all the experimental runs (Table 3). On the basis of the prescribed criteria, when Dd ≤ 0.2, the present two-stage advanced onsite system worked as an intermediate between plug-flow and completely-mixed flow reactor. For all the experimental runs of OWTS-1 and OWTS-2, the mixing pattern approached to plug flow with intermediate dispersion when the Dd ≤ 0.02. Generally, the performance of the PFR is higher than the other reactors in which dispersion number (Dd ≥ 0.2) approached to completely mixed flow reactors.[23] Therefore, the systems OWTS-1 and OWTS-2 had a greater capacity of pollutant conversion than OWTS-3 at the same hydraulic loading.
4.4. System performance 4.4.1. Hydraulic efficiency The hydraulic efficiency speculates two basic features: the ability for uniform distribution of inflow and the amount of mixing.[13,24] As mentioned above, no significant difference was observed in the dead space and mixing pattern, which indicates an even distribution of inflow in all three systems at the hydraulic loading of PFF ≤ 4. Thus, the hydraulic efficiency of the system was not significantly different and ranged from 0.76 to 0.93 at PFF 1, 2 and 4. However, it tended to decline with the decreasing porosity of the supporting media. At the hydraulic loading PFF 6, satisfactory hydraulic efficiency was observed with the values of 0.74, 0.75 and 0.70 for OWTS-1, OWTS-2 and OWTS-3, respectively. On the basis of the criteria discussed in Section 2.3.1, OWTS-1 and OWTS-2 represented a good hydraulic efficiency, whereas OWTS-3 fell into the category of good to satisfactory, in all the phases of variable hydraulic loading. The system OWTS-3 indicated a
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M.K. Sharma and A.A. Kazmi
140
COD
TSS 71.8%
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Effluent concentration (mg/L)
120 100 80.5%
80
81.2% 84.3%
85.1%
87.6%
60
78.1%
90.3%
91.2%
40
82.3% 86.7%
87.8% 90.2%
91.8%
92.1%
88.3%
20 0 A(1)
A(2)
A(4)
A(6)
C(1)
C(2)
C(4)
C(6)
Experimental run with different media at variable PFF
Figure 4. Effluent concentrations with respective removal efficiency for COD and TSS.
lower hydraulic efficiency than the other two (OWTS-1 and OWTS-2). This might be due to the lower porosity (46%) of the supporting media present in OWTS-3 than the porosity of 93% and 91% of the supporting media present in OWTS-1 and OWTS-2, respectively. Thus, the results indicated that the porosity of the media and the hydraulic loading both affected the hydraulic efficiency. On the basis of comparative analysis of the results of hydraulic characteristics of OWTS-1, OWTS-2 and OWTS-3, Aqwise carrier (ABC5) was finally selected as the packing material.
4.4.2. Treatment efficiency The performance efficiency of OWTS-1 in terms of removal of COD and TSS was found to be 90.3% and 92.1%, respectively at the steady-flow condition at 24-h HRT. It was considered as the baseline performance. At the non-steady flow condition, the removal efficiency was slightly affected as the loading increased from PFF 1 to 6. It was observed that the performance of the system significantly decreased at the hydraulic loading of PFF 6 in all the phases of the study. The overall COD and TSS removal efficiency achieved at this condition of high PFF was more than 80.5% and 82.3%, respectively. The performance of the system with baked clay media (OWTS-3) was also evaluated to develop a low-cost onsite system. Furthermore, the performances of the systems OWTS-1 and OWTS-3 were compared for relative efficiency at variable hydraulic loading of PFF as illustrated in Figure 4. For OWTS-3, the pollutant removal efficiency
in terms of COD and TSS was observed to be 71.8% and 78.1%, respectively, for the higher loading at PFF 6. When a comparison of the treatment efficiency of OWTS-1 and OWTS-3 was done, it was noted that OWTS-3 achieved approximately 6%, 8%, 7% and 11% lower COD removal efficiency at PFF 1, 2 4 and 6, respectively, than OWTS-1. Thus, the system OWTS-3 exhibited satisfactory performance with a slightly lower efficiency than OWTS-1. This might be attributed to the lower values of the physical properties of the supporting media present in OWTS-3 than the supporting media present in OWTS-1. Both OWTS1 and OWTS-3 indicated a good stability as the standard deviation of effluent COD and TSS concentration did not exceed 11.0 and 8.7 mg/L, respectively, in all the possible conditions of hydraulic loading. High treatment efficiency with a good resilience even at a high PFF was attributable to the new configuration of the present onsite system, which also enhanced the hydraulic characteristics. In addition, the appropriate physical properties of the supporting media further increased the pollutant removal efficiency.
5. Conclusions The present system showed a considerable potential to be utilized as an onsite domestic wastewater treatment system. The system showed a good to satisfactory hydraulic efficiency in all the phases of hydraulic loading conditions at PFF 1, 2, 4 and 6 with the three different supporting media (Aqwise carrier media, corrugated ring media and baked clay media) having different physical properties
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Environmental Technology (specific surface area and porosity). It was observed that there was greater impact of hydraulic loading at high PFF on the hydraulic characteristics than the physical properties of the supporting media. The treatment efficiency of the system having Aqwise carrier media (OWTS-1) was slightly higher than the system having baked clay media (OWTS-3), which was attributable to the higher porosity of Aqwise carrier media. The higher porosity of Aqwise carrier media provided greater accumulation of biomass and maximum distribution of interstitial flow, which resulted in enhancement of intermixing as well as contact between incoming substrate and microorganism. However, the performances of both the systems were highly appreciable with a good resilience during the non-steady flow conditions and the credit goes to the unique configuration of the present system, which minimized the effect of variable hydraulic shock loadings. Hence, it is suggested here that backed clay can be utilized as a better option of supporting media than Aqwise carrier media in the development of a low-cost advanced onsite domestic wastewater treatment system, especially for the economically backward rural and peri-urban areas of the developing countries. Disclosure statement No potential conflict of interest was reported by the author(s).
Funding This work is financially supported by the Ministry of Drinking Water Supply and Sanitation, Government of India, New Delhi, India through project number MRD-553-CED.
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