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Assessment of biological trickling filter systems with various packing materials for improved wastewater treatment a

b

a

a

Iffat Naz , Devendra P. Saroj , Sadia Mumtaz , Naeem Ali & Safia Ahmed a

a

Department of Microbiology, Quaid-i-Azam University, Islamabad 45320, Pakistan

b

Centre for Environmental and Health Engineering (CEHE), Faculty of Engineering and Physical Sciences, University of Surrey, Surrey GU2 7XH, UK Accepted author version posted online: 04 Aug 2014.Published online: 03 Sep 2014.

To cite this article: Iffat Naz, Devendra P. Saroj, Sadia Mumtaz, Naeem Ali & Safia Ahmed (2014): Assessment of biological trickling filter systems with various packing materials for improved wastewater treatment, Environmental Technology, DOI: 10.1080/09593330.2014.951400 To link to this article: http://dx.doi.org/10.1080/09593330.2014.951400

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Environmental Technology, 2014 http://dx.doi.org/10.1080/09593330.2014.951400

Assessment of biological trickling filter systems with various packing materials for improved wastewater treatment Iffat Naza,† , Devendra P. Sarojb,∗,† , Sadia Mumtaza,† , Naeem Alia and Safia Ahmeda∗ of Microbiology, Quaid-i-Azam University, Islamabad 45320, Pakistan; b Centre for Environmental and Health Engineering (CEHE), Faculty of Engineering and Physical Sciences, University of Surrey, Surrey GU2 7XH, UK

a Department

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(Received 2 October 2013; final version received 30 July 2014 ) Attached growth processes for wastewater treatment have significantly been improved during recent years. Their application can be extended to sustainable municipal wastewater treatment in remote locations and in developing countries for the purpose of organic matter (biochemical oxygen demand, BOD) removal and pathogenic decontamination. The aim of this study is to assess selected packing media for biological trickling filters (BTFs) and to develop a simplified model for describing the capacity of BOD removal in BTFs. In this work, BTFs with four different media viz., rubber, polystyrene, plastic and stone have been investigated at two temperature ranges of 5–15°C and 25–35°C. The average removal of both chemical oxygen demand and BOD was higher than 80 and 90% at temperature ranges of 5–15 and 25–35°C, respectively. The geometric mean of faecal coliforms in BTF using polystyrene, plastic, rubber and stone as filter media was reduced by 4.3, 4.0, 5.8 and 5.4 log10 , respectively, at a low temperature range of 5–15°C. At a higher temperature range of 25–35°C, the faecal coliform count was reduced by 3.97, 5.34, 5.36 and 4.37 log10 from polystyrene, plastic, rubber and stone media BTF, respectively. Simplified model was developed and used to estimate the optimal BOD loading rates (Bvd ) for designing robust BTF systems, with appropriate filter media. It has been concluded that highly efficient BTFs can be designed using various filter media, which may be capable of treating organic loading rates of more than 3 kg BOD/m3 day. These types of BTFs can be applied for the BOD and microbial contaminants removal of wastewater for potential reuse in developing countries. Keywords: biological trickling filters; filter media; loading rates; simplified model; wastewater treatment

1. Introduction Due to the global water crisis, more than half of the world will be facing water shortage, depleted fisheries and polluted coastlines within next 50 years.[1] Wastewater emission and inadequate management of water tend to trigger the growing human and environmental health problems throughout the world. Rapid population growth, urbanization, unsustainable water consumption practices and poor sanitation have placed immense stress on the quality as well as quantity of water resources.[2] Presently, appropriate management of wastewater is required for the improvement of public health and environment sustainability. This renders various scientific and technological challenges across the globe. In developed countries, these challenges refer to organic matter (biochemical oxygen demand, BOD) and nutrients removal, and achievement of high quality of treated effluent in order to satisfy stricter discharge standards. On the other hand, in developing economies, wastewater treatment in urban/semi urban areas is a great challenge even for the removal of BOD and microbial contaminants, for the protection of water resources, public and environment health.

Various types of biological trickling filters (BTFs) have been used for the treatment of wastewater for more than 60 years; their early forms were based on empirical understanding of biological wastewater treatment.[3] Presently, there is a growing interest in wastewater treatment using attached growth (biofilm) systems that incorporate biomass growth on support media.[4] Lower energy consumption, lower biomass production and relatively shorter treatment time are the advantages of attached growth systems over conventional activated sludge processes.[4,5] In the attached growth type of reactors, biofilm support media is one of the most critical components that provides surface to biomass growth and microbial attachment, and contact with contaminants to be removed.[6] A protocol for the design of BTF was not developed until 1950s. During 1950s and 1960s, further studies regarding the design of efficient fixed-biofilm processes were carried out by the Dow Chemical Company.[7] Numerous trickling filter process studies were conducted for developing the relationships among variables that affect the operation of BTFs. Both empirical and mechanistic mathematical models were identified for BTF process.

*Corresponding authors. Emails: [email protected]; safi[email protected] † Centre for Environmental and Health Engineering (CEHE), Faculty of Engineering and Physical Sciences, University of Surrey, Surrey GU2 7XH, UK © 2014 Taylor & Francis

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[8–10] Existing BTF process models range from simplistic empirical formulations to multidimensional numerical models. The empirical models for biofilm processes do not require the measurement of parameters such as oxygen utilization rate and mass transfer coefficients; they are not based on consideration of uniformity of biomass distribution and ideal plug flow [11] and they are not dependent on the measurement of parameters such as oxygen utilization rate and mass transfer coefficients.[12] Recently, some of the empirical models, based on the previously developed equations and formulae, have been reported.[13,14] These models have their limitations, as they are developed for specific reactor systems based on a particular data set. On the other hand, mechanistic biofilm models considered processes on micro-scale such as transport of nutrients across biofilm.[15] They demand fundamental engineering and scientific knowledge about the chemical, physical and biological processes of the system used for extrapolation. However, such knowledge is not always available to wastewater treatment process designers and operators and, often, the parameters required cannot be estimated.[16,17] Application of biofilm reactor models to actual wastewater treatment processes has some limitations due to the lack of knowledge of accurate kinetic models and uncertainty in the model parameters. For simplification of mechanistic models some parameters are often assumed to be constant. In many models, ideal plug flow has been assumed, though it has been observed to be non-ideal with increased mixing and dispersion at high flow rates.[18] This study aims at establishing a fundamental understanding of BTFs for BOD and microbial contaminants removal. The presented study is relevant to municipal wastewater treatment to produce pathogen-free water, for potential agricultural and/or recreational reuse in developing countries. Experimental data were generated from the operation of laboratory scale BTF for domestic wastewater treatment, involving different packing media materials, under two temperature ranges 5–15°C and 25–35°C. Subsequently, a simplified model was developed for the assessment of BOD removal performance of BTFs with various packing media. The experimental data on BOD removal were used to establish the simplified model that would allow improved design of BTFs for cost-effective domestic wastewater treatment.

2. Materials and methods 2.1. Experimental set-up and operating conditions A laboratory-scale BTF was built from polyvinylchloride (PVC) pipe (height of 36 , outer diameter, 14 and inner diameter, 12.4 ) of 22 L capacity. A steel cage with clamps (24 height and with diameter of 11 ) inside the pipe was used to hold filter media and has 12 L effective volume (Figure 1). The characteristics of different media used are

described in Table 1. Under-drain system (height, 8 ) with an outlet (sampling port) at a height of 3 was positioned to collect treated wastewater. A shower rose (diameter, 8 ) supported on the top of media bed distributes wastewater at flow rate of 80 mL/min. The media was seeded with activated sludge from wastewater treatment plant (Islamabad, Pakistan) for the development of the active biofilm before loading on the bioreactor for the reduction of start-up phase of the reactor. The wastewater was collected from the department of chemistry and biological sciences, Quaidi-Azam University (Pakistan); it consisted of grey water from cafeteria, black water from toilets and discharge from laboratories. A plastic container (bath room tub of 25 L capacity) was employed to hold treated wastewater and as an intermediate sedimentation tank. A water pump (power, 220 V) was used for recirculation of water. A plastic pipe (length, 125 ) connected with pump was employed to facilitate flow of water between distribution point and intermediate tank. Passive down flow aeration through a space between outer court and inner core (steel cage) was utilized to ensure aerobic conditions. With the help of water pump, 20 L of wastewater was passed through bed of BTF providing a retention time of 250 min/cycle (Figure 1). An electric dimmer connected to the water pump controlled the flow of water. The reactor was run for 48 h for each medium over a period of four months during the winter (5–15°C) and for four months during the summer (25–35°C) within two consecutive years (2011–2012). Experiments for treating wastewater in BTF were carried out involving four types of media namely, rubber, polystyrene, plastic and coarse stones. Characteristics of the media are given in Table 1. Altogether eight experiments (in triplicates) were conducted with two for each type of media at two different temperature regimes. 2.2. Physicochemical analysis of wastewater To study the performance of the reactor at both seasons (winter and summer), physicochemical analysis of the influent to each media (rubber, polystyrene, plastic and stones) and effluent at certain time intervals (12, 24, 36 and 48 h) was carried out in triplicate. The physicochemical parameters analysed included chemical oxygen demand (COD), BOD5 , dissolved oxygen (DO), total suspended solids (TSS), total dissolved solids (TDS) and electrical conductivity (EC).[19] 2.3. Microbiological analysis of wastewater Using most probable number (MPN) index, microbiological analysis for pathogenic indicators in wastewater was carried out. For the examination and enumeration of pathogen indicators such as faecal coliforms, wastewater and treated wastewater samples were incubated at 42.2°C for 24–48 h in MacConkey’s broth using multiple

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Figure 1. Schematic of laboratory scale biological trickling filter (BTF) set-up for wastewater treatment.

Table 1.

The characteristics of packing media. Media density (gm/cm2 )

Size of the media (mm)

Specific surface area (m2 m−3 )

Polystyrene Cubes Plastic Spherical

0.27 0.94

43.9 9.65

218.4 259.0

43.58 45.33

Rubber

Cubes

4.13

43.9

219.4

49.25

Stones

Oval

18.44

259.0

45.83

Media

Media shape

9.65

Voidage (%) Elemental compositiona (%) C(97.12) and O(2.88) C(53.04), Ca(0.97),N(3.05),39.96),S(1.64), Si(O.14) and Zn(1.22) C(83.5), N(2.96),O(9.49),S(0.3),Si(3.67) and Zn(0.13) C(37.69),Ca(12.33),O(48.77) and Si(1.20)

a Elemental composition of the media were determined by using X-ray Photoelectron Spectroscopy (XPS); C (carbon), O (oxygen), Ca (calcium), N (nitrogen), Zn (zinc), Si (silicon) and S (sulphur).

tube technique having inverted Durham tubes. Positive tubes were sub-cultured on MacConkey’s, Nutrient and Mannitol salt agar plates and incubated at 37 ± 2°C for 24–48 h. The colonies appeared were enumerated by colony counter and estimations were made according to the formulae; MPN/100 mL = number of colonies × dilution factor/inoculum size. Negative plates (no growth) were re-streaked before discarding tubes.

2.4.

Model development and calibration

A simplified mechanistic model for BTFs treating organic wastewater can be developed by hypothesizing that a BOD removing fixed-film bioreactor is a single limiting substrate biodegradation system, under the condition of abundant oxygen supply. Recently studied multiple substrate models, based on dynamic activated sludge modelling approach, tend to focus on

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heterotrophic–autotrophic microbial competition for the common substrate, oxygen.[3] For the purpose of BTFs that are designed for the removal of BOD and microbial contamination, the BOD removal process is the main biological process that would determine the reactor size and design loading rates. For a single-substrate (S) biodegradation, the first-order degradation of organic substrate (BOD) can be assumed as below: dS ∝ −S, dt

(1)

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where S is the main (single) substrate for the microbial growth and biofilm formation (i.e. BOD). This can be represented in the form of differential equation with a constant specific to a BTF system. dS = Kf S, dt

(2)

where K f is the biofilm reactor constant that assimilates all the reactor-specific process conditions; K f would depend on the reactor conditions such as hydraulic retention time and flow regimes influenced by the media. The above equation can be rearranged and integrated over time. The initial condition can be applied according to the laboratory experimental study, where a certain amount of substrate (BOD) is added at the start of the experiment.

 ⇒

S

S0

dS = −dtKf , S  t dS dt, = − Kf S 0

⇒ [ln S]SS0 = −Kf [t]t0 , S = − Kf t, S0 S ⇒ = e− Kf , S0

⇒ ln

⇒ S = S0 e(−Kf t) ,

Figure 2. The modelled BOD vs. time curve used for estimating an optimal design BOD loading rate (Bvd ) corresponding to an optimal BOD concentration value (S d ) and to a retention time (td ).

added only at the beginning of the experiment. Therefore, a variable organic (BOD) loading rate (Bv ) can be defined corresponding to the variable substrate (S or BOD). The re-circulation of reactor effluent is analogous to providing variable organic loading to the BTF reactor (Figure 1). QS , HA Bv HA S = . Q

Bv =

(3)

(10) (11)

And, (4) (5) (6) (7) (8)

where S 0 is the initial BOD value and S is the BOD at any time t. The initial BOD of the feed wastewater (S 0 ) can be represented in terms of volumetric loading rate (Bvo ), which is the main bulk parameter used in the design and operation of conventional trickling filters for wastewater treatment. The volumetric organic (BOD) loading rate is defined as the mass of BOD received by a BTF reactor, per unit volume, per unit time (kg BOD/m3 day). Assuming the height and area of the BTF system as H and A, respectively, and incoming flow of wastewater to be Q, the following expression can be established:   Bvo HA . (9) ⇒ Initial BOD (S0 ) = Q In the case of present experimental study, the laboratory BTF system is operated such that the substrate is

Bvo HA −Kf t . (12) e Q The above model can be used to describe the carbonaceous biodegradation kinetics in BTFs. This model can be applied to the experimental data obtained from the operation of BTFs with various media. For the purpose of model calibration, biofilm reactor is operated such that the feed wastewater is re-circulated without mixing new batch of wastewater feed throughout an experiment (48 h). Therefore, the parameter K f has been estimated by fitting the above model in the BOD vs. time data for each set of experimental results. The model calibration to obtain the value of K f , for a particular temperature range and media type, has been carried out using AQUASIM software.[3]The modelled BOD vs. time curves have been used for estimating an optimal design BOD loading rate (Bvd ) corresponding to an optimal BOD concentration value (S d ); the optimal BOD concentration (S d ) would correspond to a retention time (td ) when the exponential biodegradation starts to stabilize (Figure 2). The condition of the optimal BOD loading rate would allow sufficient retention time, and hence a good level of BOD removal without having large reactor volume. The optimal or design BOD loading rate, obtained using this approach, can be used for the design and operation of efficient BTFs constructed using various filter media. S=

5

600

750

450

600 COD (mg/L)

BOD (mg/L)

3. Results and discussion The wastewater treatment efficiency of BTF using four different media (rubber, polystyrene, plastic and stone) was compared at two temperature ranges i.e. 5–15°C and 25–35°C for various aspects, including removal of COD, BOD, TSS, TDS, EC and faecal coliform. Previously, various researchers have mentioned that about 3–60 days were required to develop biofilm in a fixed film reactor in order to initiate the wastewater treatment processes.[20–22] Furthermore, the start-up periods of fixed-film wastewater treatment reactors were reduced by inoculating activated sludge from wastewater treatment plants with suspended growth process, attached biofilm reactors or using purchased microorganisms cultured specially for enhancing wastewater treatment in fixed biofilm treatment technologies.[23,24] Similar strategy of seeding filter media with activated sludge has been followed in the present study to develop biofilm on media that was used in a trickling filter for the treatment of municipal wastewater. However, the start-up phase was reduced to 36 h at low and 24 h at mesophilic temperature regimes for achieving effective treatment of wastewater as compared to previously reported bioreactors.[20–22] Moore et al. observed shortest start-up phase of 3 days for stable operation regarding removal of COD, and Yu et al. reported start-up phase of 7 weeks for biological aerated filters in a temperature range of 20–26°C.[20,22]

300 150

450 300 150

0 0

12

24

36

0

48

0

12

24

36

48

Time (hours)

8 7 6 5 4 3 2 1 0

600 500 TSS (mg/L)

DO (mg/L)

Time (hours)

400 300 200 100 0

0

12

24

36

0

48

12

24

36

48

Time (hours)

Time (hours) 1000

900

800

750 EC (µS/cm)

TDS (mg/L)

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600 400 200

600 450 300 150

0 0

12

24

36

48

Time (hours)

0 0

12

24

36

48

Time (hours)

3.1. Performance of BTFs under various conditions 3.1.1. Removal of carbonaceous pollutants at a temperature range of 5–15°C

Figure 3. Changes in the levels of various parameters BOD, COD, DO, TSS, TDS and EC, during 48 h of treatment in the biological trickling filter (BTF) using ( ) rubber, ( ) polystyrene, ( ) plastic and ( ) stone media in the temperature range of 5–15°C.

The characteristics of the feed wastewater used for treatment in BTF at a temperature range of 5–15°C are presented in Table 2. Results of the performance of the BTFs with four different filter media at 5–15°C, over whole study period are presented in Figure 3 and Table 3. Available data indicate that BOD5 removal efficiency ranged from 86.4 to 95.7% for all media at a wastewater flow rate of 80 mL/min. The highest removal was recorded for stone media (95.7%) followed by plastic (90.9%) and polystyrene media (90.7%) as shown in Table 3. However, comparatively low BOD5 reduction was found for rubber media (86.4%). The average concentration of BOD5 dropped to 31 ± 6, 23 ± 8, 32 ± 3 and 51 ± 9 mg/L after 48 h of recirculation in

plastic, stone, polystyrene and rubber media, respectively (Figure 3). The corresponding values for COD removal ranged from 81.8 to 93.3%. The highest removal efficiency was again obtained by stone media (93.4%) followed by plastic (89.4%), polystyrene (86.3%) and rubber (81.9%) media (Table 3). After treatment of wastewater in BTF for 48 h, the mean COD concentration was dropped to 43 ± 7, 49 ± 11, 68 ± 12 and 88 ± 14 mg/L for stone, plastic, polystyrene and rubber media, respectively (Figure 3). Efficient organic removal was attributed to bio-filtrations through packing medium and biological conversion of organic matter by biofilms on the surfaces

Table 2.

The characteristics of the feed wastewater at temperature regime of 5–15°C.

Media used Polystyrene Plastic Rubber Stone

BOD5 (mg/L) 333 350 382 548

± ± ± ±

32 27 9 20

COD (mg/L) 501 468 488 653

± ± ± ±

29 14 25 16

DO (mg/L) 1.42 1.32 1.5 1.6

± ± ± ±

0.3 0.52 0.7 0.4

TSS (mg/L) 240 525 396 560

± ± ± ±

22 13 23 12

TDS (mg/L) 440 530 904 643

± ± ± ±

31 27 32 25

EC (μS/cm) 21 537 327 651

± ± ± ±

8 25 29 32

Faecal coliforms MPN/100 mL 2.29 1.32 6.5 2.52

× × × ×

104 105 105 105

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Table 3. Wastewater treatment efficiencies (%) of laboratory scale biological trickling biofilter (BTF) systems packed with different types of media at 5–15°C and 25–35°C after 48 h of operation. Treatment efficiency (%) of BTF system Temperature range 5–15°C

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25–35°C

Parameters analysed

Plastic

Stone

Polystyrene

Rubber

BOD COD DO TSS TDS EC BOD COD DO TSS TDS EC

90.9↓ 89.4↓ 80.9↑ 77.2↓ 11.5↓ 11.5↑ 94.3↓ 94.7↓ 77.1↑ 75.6↓ 19.4↑ 19.2↑

95.7↓ 93.4↓ 77.7↑ 90.1↓ 19.7↓ 19.7↑ 94.7↓ 95.9↓ 77.4↑ 74.8↓ 18.4↑ 18.5↑

90.7↓ 86.3↓ 79.3↑ 60.8↓ 58.2↑ 85.7↑ 91.4↓ 89.2↓ 70.1↑ 30.7↓ 62.9↓ 21.7↑

86.4↓ 81.9↓ 75.7↑ 55.5↓ 77.2↑ 14.9↓ 94.8↓ 93.1↓ 62.5↑ 90.5↓ 87.1↓ 26.8↑

↓; percent concentration decrease, ↑; Percent concentration increase.

of media.[25]These organic treatment efficiencies were even higher than the efficiencies reported for submerged bed reactor (40% COD removal) at a low temperature of 10°C,[26] and for trickling filter with stone wool media (94.3 ± 4.0% BOD removal) at 2.5–8.5°C.[27]This might be due to high organic loading rate (BOD and COD) applied for treatment to all media, as overall organic removal also depends on its concentrations (Table 2). The organic loading might have positive effect on biofilm growth. Wijeyekoon et al. [28] also observed increasingly high substrate loading producing increasingly thick biofilms. Several other studies indicated that temperature had a more significant influence on ammonium removal than on COD removal. They also proposed a relationship between the various parameters which characterize the wastewater such as COD, colour, turbidity, EC and TSS.[27,28] In this study, the removal of TSS was also found from influent (90.1, 77.2, 60.8 and 55.5%) by stone, plastic, polystyrene and rubber media BTF, respectively (Table 3). TSS concentration in the final effluent was recorded as 55 ± 10, 120 ± 7, 94 ± 6 and 176 ± 20 mg/L, respectively (Figure 3). The TDS concentration was increased (19.7 and 11.5%, respectively) in the final effluent of stone and plastic media TF to 801 ± 30 and 599 ± 13, respectively. And, polystyrene and rubber media BTF decreased TDS values (58.2 and 77.2%, respectively) to 184 ± 21 and 206 ± 25 mg/L, respectively, in the treated wastewater. On the other hand, EC also showed trends of increase by stone (85.8%), polystyrene (19.7%) and plastic media (11.5%). The final levels of EC recorded in the effluents were 811 ± 25, 151 ± 15 and 607 ± 36 µS/cm for stone, polystyrene and rubber media, respectively. Noticeably, rubber media BTF reduced EC (14.9%) of wastewater to 278 ± 20 µS/cm. The dissolved oxygen (DO) levels of wastewater applied to all types of media were low i.e. less than 2.0 mg/L as shown in Table 2. These low

concentrations of DO indicated high biodegradable organic load, which was easily oxidized by the dissolved oxygen in water and depleted the DO. After 48 h of treatment, effluent DO values rose to 7.18 ± 0.28, 6.94 ± 0.06, 6.85 ± 0.5 and 6.18 ± 1.1 mg/L in stone, plastic, polystyrene and plastic media TF, respectively (Figure 3). These high values of DO were due to constant recirculation and contact of wastewater with biofilm on media. 3.1.2.

Removal efficiency of faecal coliform at a temperature range of 5–15°C

Removal of pathogenic microbes is one of the main objectives of wastewater treatment, as it signifies the risk factor for public health. In the present research, faecal coliforms were chosen as pathogenic indicators. The strength of bacterial population was determined in terms of MPN/100 mL. The large microbial load in untreated wastewater, as shown in Table 2, might be due to the presence of organic pollutants and nutrients. Its application to agricultural land can cause the risk of pathogenic contamination by entering the food chain or by infecting the livestock.[29] The guidelines of WHO showed that faecal coliforms must not be detectable in 100 mL of drinking water and its recommended levels are less than 1000/100 mL and less than 105 /100 mL for unrestricted and restricted irrigation, respectively.[30] The geometric mean of faecal coliforms count in BTF using polystyrene, plastic, rubber and stone as filter media were reduced by 4.3, 4.0, 5.8 and 5.4 log10 , respectively. Furthermore, a substantial drop of 5.8 and 5.41 log10 in faecal coliforms count with residual count of 6.7 × 103 and 8.06 × 102 MPN/100 mL have been observed in the final effluent from rubber and stone media BTF, respectively. Comparatively less removal of pathogenic indicator was noticed in the treated wastewater stream from polystyrene and plastic media BTF with residual count of 7.0 × 102

3.1.3.

Removal of carbonaceous pollutants at a temperature range of 25–35°C

Results of the performance of the BTF with four different filter media at 25–35°C, over whole study period are presented in Figure 4 and Table 3. The initial feed loads on average during these studies are presented in Table 4. The percentage removal efficiency of COD from wastewater by all media under study was in the range of 90% after 48 h of recirculation in BTF. Highest removal was shown by stone media (95.9%) followed by plastic (94.7%), rubber media (93.1%) and polystyrene media BTF (89.3%) at constant hydraulic flow rate of 80 mL/min. The levels of COD was decreased from 821 ± 23, 1351 ± 117, 1016 ± 29 and 360 ± 32 mg/L to 32 ± 17, 69 ± 23, 69 ± 16 and 38 ± 14 mg/L by stone, plastic, rubber and polystyrene media BTF, respectively. While, for BOD removal rubber media BTF showed highest efficiency (94.8%) followed by stone (94.7%), plastic (94.3%) and polystyrene media (91.4%) (Table 3). The BOD reduction was recorded from 558 ± 37, 918 ± 45, 816 ± 63 and 168 ± 31 mg/L to 28 ± 9, 52 ± 16, 41 ± 19 and 14 ± 5 mg/L by stone, plastic, rubber and polystyrene media BTF, respectively (Figure 4). However, Rajeb et al.

7

800

1200 COD (mg/L)

1400

BOD (mg/L)

1000

600 400

1000 800 600 400

200

200

0

0 0

12

24

36

48

0

12

24

36

48

Time (hours)

Time (hours) 1000

9 8 7 6 5 4 3 2 1 0

800 TSS (mg/L)

DO (mg/L)

600 400 200 0

0

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0

Time (hours) 1800 1500 1200 900 600 300 0 0

12

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Time (hours)

12

24

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Time (hours)

Ec (µS/cm)

and 6.52 × 102 MPN/100 mL, respectively. These results indicated that BTF system with any of these four growth supporting media was more effective for the removal of faecal coliforms as compared to other aerobic systems reported [25,31] even at low temperature (5–15°C). The significantly higher performance of all filter media might be due to enhanced adhesion of these microbes in the slim layers (extracellular polymer) in the form of biofilms or due to retention on the filter media by adsorption,[32] and later removal or deactivation by predation or natural die off process.[33] However, the adsorption of bacteria has been observed substantially greater at high than at low temperatures.[32] The work of Von Sperling and Chernicharo [34] reported that, depending on the type of process and the operational conditions, aerobic treatment provides about 1–2 log pathogens removal. Mahmoud et al. [25] reported that down-flow hanging sponge (DHS) reactor having polyurethane as media has proved promising for 4.4 log10 reduction in faecal coliforms. During biological treatment of wastewater, the removal efficiency of pathogenic indicators also depends on DO, pH, temperature and the efficiency in removing suspended solids.[35] Fernandez et al. [36] have correlated increased DO with bacterial die-off in aquatic environment. The pathogenic indicator bacteria (allochthonous) usually require organic matter.[37] After fixation on media, microbial biofilm contributed in the processes of removal of pollutants from wastewater by oxidation, adsorption, absorption, etc. The capability of fixed biofilm reactors to treat wastewater is directly proportional to biomass concentration in it.[38]

TDS (mg/L)

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900 800 700 600 500 400 300 200 100 0 0

12 24 36 Time (hours)

48

Figure 4. Changes in the levels of various parameters BOD, COD, DO, TSS, TDS and EC, during 48 h of treatment in biological trickling filter (BTF) using ( ) rubber, ( ) polystyrene, ( ) plastic and ( ) stone media in the temperature range of 25–35°C.

[39] reported similar BOD, COD, SS removal of 80, 91.6 and 43%, respectively, from wastewater by using aerobic filter. These higher organic pollutants removal were due to higher temperature.[12,40] El-Monayeri et al. [41] noticed that increase in temperature from 15 to 31°C increased the COD and BOD removal ratios by biofilters. Corresponding DO levels increased with constant recirculation form 62 to 78% by different media; highest DO concentration was achieved by plastic (8 ± 0.22 mg/L) followed by polystyrene (7 ± 0.98 mg/L), stone (7 ± 0.45 mg/L) and rubber media BTF (7 ± 0.4 mg/L). Increase in DO has been generally related to diminution carbonaceous matter and parameters, such as COD, BOD5 and TSS and TDS, during the treatment of wastewater through attached growth systems.[42] Sá and Boaventura [43] also reported that COD/BOD removal was positively correlated with increase in DO levels. These results are in line with reduction in TSS levels, the highest removal of TSS concentration was shown by rubber media BTF from 738 ± 17 to 70 ± 17 mg/L (90.5%). While the lowest

8

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Table 4. The characteristics of the feed wastewater at temperature regime of 25–35°C. Media used

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Polystyrene Plastic Rubber Stone

BOD5 (mg/L) 168 918 816 558

± ± ± ±

31 45 63 37

COD (mg/L) 360 1351 1016 821

± ± ± ±

32 117 29 23

DO (mg/L) 2.24 1.8 2.7 1.63

± ± ± ±

0.8 0.8 0.8 0.5

TSS (mg/L) 978 422 738 437

TSS removal was recorded for polystyrene media from 978 ± 41 to 677 ± 52 mg/L (30.7%). However, plastic (75.6%) and stone media BTF (74.8%) showed almost similar performance and reduced TSS from 422 ± 24 and 437 ± 34 mg/L to 103 ± 16 and 110 ± 17 mg/L, respectively (Figure 4, Table 3). Correspondingly, polystyrene and rubber media BTF decreased TDS levels from 124 ± 26 and 1724 ± 90 mg/L to 46 ± 24 and 222 ± 35 mg/L (62.9 and 87.1%), respectively, after 48 h of treatment (Figure 4). Microbial population developed on these media were presumably responsible for the removal of TSS and TDS from wastewater during recirculation in the reactor by aerobic and anaerobic metabolic process. And, plastic and stone media BTF increased TDS concentration from 618 ± 20 and 643 ± 25 mg/L to 767 ± 35 and 771 ± 39 mg/L (19.4 and 18.4%), respectively. These results are in consistence with increasing trend of EC during the treatment. The values of EC showed increase for all media type i.e. plastic, polystyrene, rubber and stone media from 625 ± 86, 378 ± 24, 122 ± 36 and 635 ± 58µS/cm to 774 ± 53, 483 ± 40, 168 ± 44 and 779 ± 47µS/cm (19.2, 21.7, 26.8 and 18.5%), respectively, after whole treatment period (Figure 4, Table 3). It clearly reflected the release of ions and dissolve components from degradation of large suspended chemical compounds during treatment.

3.1.4.

Removal efficiency of faecal coliform at a temperature range of 25–35°C From the initial microbial load at 25–35°C in municipal wastewater (Table 4), the geometric mean of faecal coliforms count in the effluent was reduced by 3.97, 5.34, 5.36 and 4.37 log10 from polystyrene, plastic, rubber and stone media BTF, respectively. A highly significant drop was observed in both rubber and plastic media BTF effluents with residual count of 5.88 × 102 and 5.3 × 102 MPN/100 mL, respectively. Relatively less significant drop of 3.97 and 4.37 log10 was noticed in the effluents from polystyrene and stone media BTF with residual count of 6.2 × 102 and 7.91 × 102 , respectively. However, these results were almost same as recorded at low temperature regime of 5–15°C, but more efficient than other aerobic systems. Abdeen et al. [44] found 2.2 log removal of faecal coliform for saline sewage treatment by anoxic and aerobic reactors. However, less than 1.6 log

± ± ± ±

41 24 17 34

TDS (mg/L) 124 618 1724 629

± ± ± ±

26 20 90 36

EC (μS/cm) 378 625 122 635

± ± ± ±

Faecal coliforms MPN/100 mL

24 86 36 58

9.53 2.21 2.35 2.44

× × × ×

103 105 105 104

removal of indicators faecal contamination was reported from wastewater by an aerobic reactor by Rajeb et al. [39]. Polystyrene and stone media BTF showed less number of pathogen removal might be due to low strength (9.53 × 103 MPN/100 mL) of faecal coliforms in raw wastewater (Table 3). The adsorption of bacteria on porous media is influenced by its concentration and its survival with increase of temperature.[32] The final effluent can be used for unrestricted (less than 1000 faecal coliforms/100 mL) and restricted (less than 105 faecal coliforms /100 mL) irrigation.[30] The bacterial count drastically decreased with removal of BOD, COD, TSS, etc. during treatment in all types of media used during study. Der Steen et al. [45] reported the removal of BOD could result in the simultaneous reduction of coliform populations. A study on correlation matrix indicated that TSS positively correlates with faecal coliform concentration in dairy wastewater using an ecological treatment system.[46]Though, Yathavamoorthi et al. [47] observed a positive correlation of faecal coliform with BOD than with suspended solids.

3.2.

Application of simplified model

The core purpose of developing simplified model was to apply it for optimal design and operation of BTF systems for BOD removal, as it is still a challenge in the developing world. The simplified model does not require rigorous calibration and computational effort needed in the case of complex mechanistic models, and it takes into account the effects of factors related to the biodegradation process and reactor design. Thus, by means of the simple model, a direct comparison of different media performance, at different temperature ranges of 5–15°C and 25–35°C, can be made. It gives a simple method to ascertain ideal media type for designing a wastewater treatment process, and compact BTFs required for BOD removal at specific temperature ranges. The value of K f relates to the overall performance of BTF in removing BOD, and it takes into account the properties of media at specific temperature ranges. Figures 3 and 4 show a typical BOD concentration profile obtained over a period of 48 h of treatment at low and mesophilic temperatures, respectively. To estimate the values of K f , eight sets (in triplicates) of results, four for each temperature regime, were used. The model application to the BOD data sets resulted in the values of K f for each filter media in a specific reactor configuration. The R2 value calculated

Environmental Technology Table 5. Biological trickling filter parameters at low temperature range (5–15°C). Kf

Sd (kg/m3 )

Bvd (kg/m3 day)

td (h)

0.04 0.05 0.03 0.04

0.19 0.25 0.20 0.48

1.9 1.9 1.5 5.0

12.9 11.4 16.5 12.0

600 500

BOD (mg/L)

Plastic Stone Polystyrene Rubber

(b)

600 500

BOD (mg/L)

Media

(a)

9

400 300 200 100

400 300 200 100

0

0 0

0.05 0.07 0.04 0.04

Sd

0.41 0.17 0.99 0.48

Bvd

3.1 1.3 1.2 3.7

day)

td (h) 14.0 18.0 12.8 11.7

500

500 BOD (mg/L)

Plastic Stone Polystyrene Rubber

Kf

(kg/m3

400 300 200 100

12 24 36 Time (hours)

48

400 300 200 100

0

0 0

12 24 36 Time (hours)

48

0

12 24 36 Time (hours)

48

Figure 5. BOD removal by (a) polystyrene, (b) plastic, (c) rubber and (d) stone media biological trickling filter (BTF) in the temperature range of 5–15°C; (•) experimental data and (–) simulated results.

(a)

(b)

200

1000

160

800

BOD (mg/L)

BOD (mg/L)

120 80 40

600 400 200

0

0 0

(c)

12 24 36 Time (hours)

48

0

12

24 36 Time (hours)

48

0

12

24 36 Time (hours)

48

(d)

600

1000

500

800 BOD (mg/L)

from the results of these experiments ranges between 0.888 and 0.994 at a low temperature range of 5–15°C; its value varies from 0.803 to 0.954 for all media types at mesophilic temperature range of 25–35°C. The values of K f obtained from experimental results indicate a better performance for rubber media, followed by plastic and stone media at low temperature conditions. In the higher temperature range, the values of K f relate to the highest removal of BOD by rubber media, followed by plastic and stone media. Thus, BTF with rubber, plastic and stone media can be employed for the treatment of high BOD loads in the areas having two temperature regimes from low to mesophilic. The constant K f has a direct relationship to the removal of carbonaceous matter (BOD and COD) from the wastewater and indicate the active biofilm concentration within the reactor. The values of K f are specific to the media, under specific temperature range and hydraulic conditions. The simulated results from the model (Figures 5 and 6) showed slight difference in the values of constant between different media at different temperatures (Tables 5 and 6). The values of K f , estimated for each media, were almost similar at same temperature range. A slight variation between media can be expected in their density, chemical composition, texture and shape. The difference in K f values may be ascribed to this, and its resulting effects on liquid flow through the reactor due to differences in the void geometries (Table 1). It has subsequent effect on biofilm composition and attachment to media surface. Furthermore, a slight variation towards higher side was noticed in the K f values at higher temperatures (25–35°C). Results indicated that rubber media has almost same Kf values, and it is capable of handling high organic loadings at both temperature ranges (Tables 5 and 6; Figures 5 and 6). Moreover, plastic media also has almost same K f values (0.04 and 0.05) at both temperature ranges, but capable of removal of higher BOD load (3.1 kgBOD/m3 day) at

0

(d) 600

BOD (mg/L)

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Media

(kg/m3 )

48

(c) 600 BOD (mg/L)

Table 6. Biological trickling filter parameters at mesophilic temperature regime (25–35°C).

12 24 36 Time (hours)

600 400 200

400 300 200 100 0

0 0

12

24

36

Time (hours)

48

Figure 6. BOD removal by (a) polystyrene, (b) plastic, (c) rubber and (d) stone media biological trickling filter (BTF) in the temperature range of 25–35°C; (•) experimental data and (–) simulated results.

higher temperatures. It indicates the positive effect of temperature on development and composition of biofilm on plastic media; however, there was difference in the K f values for stone (0.05 and 0.07) and polystyrene (0.03 and 0.05) media at temperature ranges of 5–15°C and 25–35°C.

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It has slightly lower Kf value at 5–15°C, and comparatively capable of treating slightly higher BOD load (1.9 and 1.5 kg BOD/m3 day) than mesophilic temperature (1.3 and 1.2 kg BOD/m3 day) with Kf value of 0.06 and 0.04 for stone and polystyrene media, respectively. In summary, most of these media appear to be capable of handling much higher BOD loading rates, as compared to conventional fixed film trickling filters that are designed for less than 1 kg BOD/m3 day for the purpose of BOD removal.[3] Therefore, economical BTFs with compact volume can be designed, by using the studied filter media types, for wastewater treatment and reuse. These findings are highly relevant to developing countries where BOD and microbial contaminant removal is of primary concern for the purpose of water reuse in agriculture. Furthermore, BTFs with the studied media types may similarly demonstrate efficient nitrification performance. This opens up an opportunity to conduct future study on the nitrification potential of such BTFs that may be designed for higher nitrogen and BOD loading rates. 4. Conclusions In the present study, the performance of BTF using four media (rubber, plastic, polystyrene and stone) at two temperature ranges, 5–15°C and 25–35°C, for municipal wastewater showed that all the media used in BTF produced good quality effluent regarding carbonaceous matter (BOD) and faecal coliforms removal. The wastewater treatment efficiency increased with treatment time from 12 to 48 h and average BOD removal values obtained from stone, plastic, polystyrene and rubber media were 93.4, 89.4, 86.3 and 81.9%, respectively, at 5–15°C. While higher BOD removal was observed at a temperature range of 25–35°C in all the cases giving the highest BOD efficiency of rubber media BTF (94.85%). With regard to faecal coliform, the effluent produced from four different media BTF, at low and mesophilic temperature, complied with the guidelines regarding the use of treated wastewater for agriculture use. The decrease was to a level of < 1000 MPN/100 mL in all the treatments except rubber media BTF at low temperature which was reduced to 6.7 × 103 MPN/100 mL. The reduction in MPN for faecal coliform was higher at mesophilic temperature range as compared to low temperature range. From the BOD vs. time data a simplified model was developed and calibrated for selected media types and temperature ranges. This provided a robust tool to design and operate BTFs with BOD loading rate of higher than 1.5 kg BOD/m3 day by using plastic, stone and polystyrene BTFs. Moreover, rubber media BTF can be designed for higher BOD removal of 5 kg BOD/m3 day at a low temperature regime of 5–15°C. At mesophilic temperature ranges of 25–35°C, BTFs with polystyrene and stone media can be designed for the loading rate higher than 1.2 kg/m3 day; plastic and rubber media BTFs can treat the BOD loading

rate higher than 3 kg BOD/m3 day. This approach would be most applicable in designing high efficiency bioreactor with fixed biofilm growth, by utilising locally available filter media. Therefore, BTF systems have high potential to be scaled up for large-scale application for wastewater treatment in the developing countries. Further research should be conducted to assess the potential of such BTFs in efficient nitrification at high nitrogen and BOD loading rates. Acknowledgements Authors sincerely acknowledge the International Research Support Programme (IRSIP) of Higher Education Commission of Pakistan (HEC, Pakistan), which supported I.N. to work at University of Surrey (UK) as part of her PhD research. This research work is a part of a project ‘Small Scale Sewage Treatment and Wastewater Reuse System for Pakistan’ funded by HEC under Pak/US academic research program.

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