Environmental Technology

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Influence of wastewater treatment plants' operational conditions on activated sludge microbiological and morphological characteristics Elisavet Amanatidou, Georgios Samiotis, Eleni Trikoilidou, Dimitrios Tzelios & Avraam Michailidis To cite this article: Elisavet Amanatidou, Georgios Samiotis, Eleni Trikoilidou, Dimitrios Tzelios & Avraam Michailidis (2016) Influence of wastewater treatment plants' operational conditions on activated sludge microbiological and morphological characteristics, Environmental Technology, 37:2, 265-278, DOI: 10.1080/09593330.2015.1068379 To link to this article: http://dx.doi.org/10.1080/09593330.2015.1068379

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Environmental Technology, 2016 Vol. 37, No. 2, 265–278, http://dx.doi.org/10.1080/09593330.2015.1068379

Influence of wastewater treatment plants’ operational conditions on activated sludge microbiological and morphological characteristics Elisavet Amanatidou ∗ , Georgios Samiotis, Eleni Trikoilidou, Dimitrios Tzelios and Avraam Michailidis Environmental Chemistry and Wastewater Treatment Lab, Environmental Engineering and Pollution Control Department, Technological Education Institute of Western Macedonia, Koila, Kozani 50100, Greece

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(Received 27 March 2015; accepted 26 June 2015 ) The effect of wastewater composition and operating conditions in activated sludge (AS) microbiological and morphological characteristics was studied in three AS wastewater treatment plants (WWTPs): (a) a high organic load slaughterhouse AS WWTP, operating at complete solids retention, monitored from its start-up and for 425 days; (b) a seasonally operational, low nitrogen load fruit canning industry AS WWTP, operating at complete solids retention, monitored from its start-up and until the end of the season (87 days); (c) a municipal AS WWTP, treating wastewater from a semi-combined sewer system, monitored during the transitions from dry to rainy and again to dry periods of operation. The sludge microbiological and morphological characteristics were correlated to nutrients’ availability, solids retention time, hydraulic retention time, dissolved oxygen, mixed liquor suspended solids (MLVSS), organic load (F/M) and substrate utilization rate. The AS WWTPs’ operation was distinguished in periods based on biomass growth phase, characterized by different biological and morphological characteristics and on operational conditions. An anoxic/aerobic selector minimizes the readily biodegradable compounds in influent, inhibiting filamentous growth. Plant performance controlling is presented in a logic flowchart in which operational parameters are linked to microbial manipulation, resulting in a useful tool for researchers and engineers. Keywords: biomass growth phases; COD:N:P limitations; filamentous growth; WWTP operational parameters; activated sludge characteristics

Introduction Operating conditions in biological wastewater treatment plants (WWTPs) have generally been developed empirically, on the basis of around 100 years’ experience with conventional activated sludge (AS) and biofilm processes. Only over the past few decades has this empirical approach increasingly been supplemented by a mechanistic understanding of the growth rates and physiology of the organisms concerned. Experience from the operation of treatment plants has shown that system performance is reproducible and consistent under constant operating conditions.[1] Microbial community composition and biodiversity in biological WWTPs are developed dynamically and not necessarily predictably, as a function of the operating conditions.[2] The most common organisms that participate in the biodegradation of organic material in the biological system for effluent treatment are bacteria, protozoa and metazoa. The organisms most directly involved in wastewater treatment are the bacteria. They dominate all other groups, in number and biomass, and affect the process of mineralization and elimination of organic and inorganic nutrients (decomposers). The other biotic components, protozoa and metazoa (eukaryotic organisms), feed on dispersed bacteria and other organisms.[3]

*Corresponding author. Email: [email protected] © 2015 Taylor & Francis

The presence of protozoa and metazoa in biological wastewater treatment processes was observed almost as soon as each process was introduced, but the significance of these microorganisms has been only recently underlined. It has been estimated that protozoan biomass can reach values of 250 mg L–1 (dry weight), constituting over 9% of volatile solids.[4] Protozoa play a secondary but important role in wastewater system purification. The positive effects of protozoa on carbon mineralization by bacteria in AS are well known. The presence of particular types of protozoa is related to effluent quality and plant performance, so they can be used as bioindicators. Protozoa in the AS treatment process fall into four main classes: amoebae, flagellates, ciliates (freeswimming, crawling and stalked) and sporozoa (Apicomplexa spp.). Metazoa are separated into rotifers, nematodes (sub-class) and Oligotrichia such as Aeolosoma spp.[5] Seventy per cent of the protozoa species in sludge are ciliates.[6–8] Ciliated protozoa in sewage treatment plants produce clear effluents of good quality because of their ability to feed on bacteria and suspended particles and to induce flocculation. They are very sensitive to environmental variations and are recognized as indicators of the operating conditions of plants.[6–8] This also applies for the metazoan species, such as the rotifers, which can be

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found only in very stable AS environments with low F/M ratios.[9] Most ciliates present in biological WWTPs can be divided into three main groups according to their feeding behaviour: free swimmers, which swim in the sludge liquid fraction and remain in suspension in the sedimentation tank; attached ciliates, which are attached to the bacterial aggregates and settle in the sedimentation tank; and crawlers, which live in the floc surface and settle in the sedimentation tank. Populations of organisms associated with the flocs are at great advantage compared to those which swim freely in the liquid fraction and thus can be washed out of the system through the effluent. Furthermore, free swimming and attached ciliates are food competitors for dispersed bacteria whereas the crawling ciliates, with a ‘ventral mouth’, feed on the lightly adherent bacteria of the floc surface, living in an exclusive ecological niche. In healthy established AS, the latter are therefore prevalent.[8] A fully functioning plant need not host species characteristic of one of the colonization phases, unless dysfunctions, due to the amount of sludge, the degree of aeration, the sewage retention time and the organic load at the input, cause regression in the environmental conditions.[10] For example, crawling ciliates (hypotrichs) decrease with increasing organic loading (no hypotrichs are observed in sludge loaded above 0.6 kgBOD5 kg MLSS−1 day−1 ), while attached ciliates (peritrichs) are able to grow over a large range of sludge loadings.[11] The protozoan community may affect the whole food web of the microbial ecosystems, thus affecting the biological performance of WWTPs. Studies of the relationships between protozoa and physicochemical and operational parameters have revealed that the species structure of these communities is an indicator of plant efficiency.[3] In modern systems, where there is a low load and high sludge retention time, the presence of protozoa such as ciliates, flagellates and amoebae, or even small metazoa, is very common. It is generally assumed that their primary role in wastewater treatment is the clarification of the effluent.[3,12] Employing any strategy for sludge reduction has an impact on the microbial community (e.g. microbial population shift) in AS processes, which may influence the sludge settling and dewatering, and the effluent quality. In AS processes considerable amounts of biosludge are formed and thus, designing for low loadings is costly since it requires either large reactor volumes or sophisticated means, such as membrane separation units, in order to maintain an extraordinarily high biomass concentration in the process. There is the possibility of minimizing sludge production in aerobic wastewater treatment through manipulation of the ecosystem so that most of the bacterial biomass produced is consumed by predating protozoa and metazoa. It is well known that the biological characteristics of AS are linked to the operating conditions of a WWTP and microbial

manipulation influences its treatment efficiency and net sludge production.[13–16] Generally, operating conditions that affect AS morphological and biological characteristics in biological treatment, selected in such a way that desired bacteria are accumulated, are the MLSS concentration, the food to microorganisms ratio (F/M), the substrate utilization rate (SUR), the solids retention time (SRT), the HRT, the DO and the return activated sludge (RAS) rate. Additionally, wastewater-dependent parameters such as volumetric and nutrients load can affect AS characteristics such as floc cohesiveness and microbial species diversity.[16,17] In a given WWTP, higher SRT increases MLSS concentration and decreases F/M and biomass growth, provided that influent organic and nutrient content are constant and sufficient for biological treatment.[16,18] According to literature, BOD5 :N:P (5 days biochemical oxygen demand to nitrogen to phosphorus ratio) and COD:N:P (chemical oxygen demand to nitrogen to phosphorus ratio) ratios of 100:5:1 and 150:5:1 are proposed to define the minimum nutrient requirements of AS systems designed for total carbon removal. It should be noted, however, that although specific values for these ratios may be useful as a rough index, they may not accurately indicate specific requirements of the AS for a given operating condition. For each operation, the nutritional demand of the microorganisms is closely related to the net amount of biomass produced, which is reflected by the observed yield (Yobs ) in an AS WWTP.[19] The importance of DO concentration in AS characteristics is widely known. It is one of the major parameters for microbial manipulation and can greatly affect floc’s cohesiveness and extracellular polymeric substance (EPS) content.[20] The high oxygen concentration processes increase the oxygen content within the AS floc, where usually anoxic conditions prevail; thus aerobic utilization of EPS results in floc volume minimization and thinner floc formation. The increased oxygen concentration in the mixed liquor leads to a deep diffusion of oxygen, which subsequently enlarges the aerobic volume inside the resulting flocs and the oxygenated microorganisms in the floc matrix are degraded.[21] Additionally, the high DO assists in the growth and maintenance of predator protozoan and metazoan species, which contribute to lower net biomass production and excess sludge minimization. According to Amanatidou et al. [16] at full-scale AS WWTPs, complete solids retention, extended aeration, high DO and high MLVSS lead to extended starvation conditions and predator species growth. In such systems, net sludge production tending to zero can be achieved. Low substrate availability, nutrient deficiency and low DO concentration in aeration compartments can result in biomass growth inhibition, unwanted microbial species growth, such as filamentous bacteria that compromise the effluent quality of an AS WWTP.[22] In addition, high readily biodegradable substrates favour filamentous

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Environmental Technology bacteria growth such as Sphaerotilus natans, Type 1701, Haliscomenobacter hydrossis, Thiothrix spp., Type 021N, Type 1851, Nostocoida limicola spp. Generally, at high substrate concentration processes, preventive actions to avoid filamentous growth seem to agree that readily biodegradable (rb) substrates need to be consumed. Aerobic, anoxic or anaerobic selectors have been used in order to minimize the rbCOD, which in most cases results in controlling the filamentous growth.[2,23] Excessive filamentous bacteria have a great negative impact on sludge settling properties and thus it is essential to understand the cause of their presence and prevent or resolve sedimentation problems. Some few of filaments make flocs stronger and compact macrostructure, but more than a few of them cause a bulking problem, increase surface area of the flocks and decrease the density of the flocs.[22] At high SRT, alterations of floc morphology (e.g. pinpoint flocs) and growth of unwanted bacterial species, such as filamentous, compromise sedimentation efficiency and effluent quality, especially in membraneless AS processes.[15,19] By applying a combination of high SRT, high DO in aerobic compartment, high HRT, high RAS rates, low F/M ratios and low SUR values, high MLSS results and, microbial manipulation is achieved. When no sludge is disposed from an AS system, the actual SRT is referred to as complete SRT, where new cells’ production rate becomes equal to the decay rate and predation phenomena and MLSS reach steady-state conditions (plateau phase).[16,24–28] In the plateau phase low-growth rates result and MLVSS to MLSS ratio is maintained relatively constant. In these operating conditions, growth and maintenance of predator species, minimization of sludge production and efficient wastewater treatment are assured. Good settling efficiency is obtained by high RAS rate causing forced sedimentation of sludge in clarifiers. Usually, at high RAS rates, flocs may scatter, resulting in a turbid effluent. If no filaments are present then this problem is reversible because of the sludge’s natural tendency to aggregate into bigger flocs.[13,15,29] The aim of this work is to study the effect of different wastewater compositions and WWTPs’ operational conditions on AS morphological and microbiological characteristics. Nutrients availability, SRT, DO, MLSS, MLVSS, F/M, SUR and WWTP design are linked to specific AS morphological characteristics and to the presence of indicative microbial species. Operational parameters were linked to microbial manipulation and to problems and symptoms in plant performance through a practical way of WWTP controlling. Materials and methods In this work, the effect of wastewater composition and operating conditions in AS microbiological and morphological characteristics was studied in three AS WWTPs): (a) a high organic load slaughterhouse AS WWTP,

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operating at complete solids retention, monitored from its start-up and for 425 days; (b) a seasonal operation, low nitrogen load fruit canning industry AS WWTP, operating at complete solids retention, monitored since its start-up and until the end of the season (87 days); (c) a municipal AS WWTP, treating wastewater from a semi-combined sewer system, monitored during the transitions from dry to rainy and again to dry period of operation. In all AS WWTPs, the sludge microbiological and morphological characteristics were correlated to nutrients availability (COD:N:P), SRT, HRT, DO, MLSS, MLVSS, F/M and SUR. Wastewater characteristics of these three treatment plants are presented in Table 1. The slaughterhouse WWTP is installed in the Prosotsani municipality of Drama Prefecture in Central Macedonia, Greece. This WWTP integrates a preliminary, simultaneous nitrification/denitrification process and a preanoxic denitrification, complete mix, extended aeration AS system with complete retention of solids. The effluent of the preliminary system overflows in the extended aeration system, while excess sludge is transferred periodically from the preliminary process into the extended aeration process. An initial removal of COD and TN is achieved in the preliminary system and the main treatment takes place in the pre-anoxic denitrification, complete mix, extended aeration AS system. In this work, the different AS biological and morphological characteristics in three distinguished operation periods were studied: (a) first period, characterized by high biomass growth rates and young sludge, (b) second period, characterized by lower growth rates and mature sludge and (c) third period, characterized by almost zero net biomass production, old sludge and stabilized WWTP operation. The fruit canning industry’s WWTP is installed in Drama industrial area, Central Macedonia, Greece. It consists of an aeration tank for organic compounds’ oxidation and nitrification. The effluent is further treated in the Drama industrial area biological treatment facility. Two distinguished operation periods were studied: (a) the first period, considered as the lag phase, characterized by low MLSS and MLVSS concentrations, very low biomass production and poor treatment efficiency. (b) The second period, considered as the growth phase, characterized by high biomass growth rates and sufficient wastewater treatment. Due to the seasonal operation of the fruit canning industry, about three months’ working period, the AS process was terminated before reaching steady-state conditions (stabilization phase). The municipal WWTP, treating wastewater from a semi-combined sewer system, was installed in Paralimni municipality at Southeast Cyprus and includes all the necessary steps for biological removal of organic, nitrogen and phosphorus compounds. Based on the changes in the AS characteristics, observed during six continuous days of rainfall, separation into three distinguished operation periods was conducted: (a) the first period, characterized by

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Wastewater characteristics of the studied WWTPs. High-strength industrial wastewaters

Wastewater characteristics

Slaughterhouse WWTP

Fruit canning WWTP

Influent

Influent

Average flow (m3 d−1 )

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Domestic wastewater

Effluent 80

Effluent

4320

Average COD (mg L−1 )

4150

36

3415

80c

Average BOD (mg L−1 )

2380

20

2117

57c

Average SS (mg L−1 )

470

< 30

160

33c

Average TN (mg L−1 )

272

16.5

Average TP (mg L−1 )

51

Conductivity (µS cm−1 ) Average wastewater COD:N:P ratio

9.7

< 2c

0.5

6

< 1c









150:9.8:1



150:0.33:0



Municipal WWTP Influent

Effluent

4280a 5350b

1110a < 20a 990b 39b a 615 < 20a 535b 25b a 190 < 20a 150b 21b a 42 5.59a 27b 5.14b 10a 0.28a 5.2b 0.19b 2,866a 2,610a 2,300b 2,125b 150:5.69:1.37a 150:3.2:0.75b

a Average value during dry season. b Average value during rainy season. c Average value after lag phase.

dry weather, relatively constant COD concentration, sufficient nutrient content, high conductivity and relatively constant biomass concentration and volumetric load. High conductivity is attributed to high wastewater salinity and low precipitations in the island. (b) The second period, characterized by rainy weather, resulting in lower COD concentration, limited nutrient content, lower conductivity and higher volumetric load. (c) The third period, characterized by dry weather following the rainfall, resulting in an increase of COD, of nutrient content and conductivity, and in lower volumetric load. Physicochemical and biological monitoring of slaughterhouse and fruit canning WWTPs was performed by taking samples from influent, mixed liquor and effluent. Samples were obtained according to ISO 5667-10:1992 (Water quality sampling, part 10: Guidance on sampling of wastewaters), conveyed to the laboratory within 2–4 h, stored at a temperature under 4°C and analysed within 24 h. Determination of COD, BOD5 , total Kjeldahl nitrogen (TKN), total nitrogen (TN), DO, sludge volume index (SVI), total suspended solids (TSS) and total volatile suspended solids was carried out at the accredited research facility (name deleted to maintain the integrity of the review process), by using the procedures outlined in the 22nd edition of Standard Methods for the examination of water and wastewater. Microscopic analysis for the AS morphological and microbiological characteristics was performed using a Leica DM 1000 phase contrast microscope at X40 to X1000 magnifications, within 4 h of sample collection. AS photographs were acquired with a Leica DFC295 digital microscope camera. The samples’ analysis

from Paralimni municipal WWTP was conducted in the laboratory of the facility by applying standard methods. The identification of microbial species and the observation of AS morphology of Paralimni WWTP were performed using a Kruss MBL2000 optical, light microscope with X40 up to X1000 magnification, within 2 h of sample collection. Furthermore, the operational parameters, such as F/M, SUR, SRT and HRT, were calculated in accordance with the guidelines specified in international literature.

Results and discussion Slaughterhouse complete solids retention AS WWTP The slaughterhouse’s wastewater composition was similar to that of similar size slaughterhouses studied by [30,31]. Relative small deviations in influent-measured characteristics were observed due to the use of an influent equalization tank. The average BOD5/COD ratio was approximately 0.57, lower than typical domestic wastewaters (0.6–0.7). The high BOD20/COD and BODultimate /COD ratios of 0.91 and 0.98, respectively, indicate the high biodegradability of the slowly hydrolysable wastewater substrate.[15] Furthermore, the mean COD:N:P ratio in the influent was 150:9.8:1.8, indicating sufficient nutrient content with a relatively high concentration of nitrogen compounds. The WWTP operational parameters are presented in Table 2. The preliminary, simultaneous nitrification/ denitrification system operated at a relative constant MLSS concentration of 5 g L−1 and at relatively low DO levels (0.2–0.8 mg L−1 ), high SRT (approximately 29 days), high

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Table 2. The WWTP’s operational parameters. Slaughterhouse pre-anoxic extended aeration WWTP

Fruit canning WWTP

Municipal WWTP

>4

>4

MLSS (mg L−1 ) MLVSS (mg L−1 ) SRT (days) F/M (kgBOD5 KgML VSS−1 d−1 )

Varied (see Figure 1) Varied (see Figure 1) Complete retention Varied (see Figure 1)

Varied (see Figure 2) Varied (see Figure 2) Complete retention Varied (see Figure 2)

SUR (kgCOD Kg VSS−1 d−1 )

Varied (see Figure 1)

Varied (see 2)

HRT (days)

1.25 (pre-anoxic tank)

1.39

3.8a 6.1b 5150–5900 3910–4660 13 0.29a 0.38b 0.29a 0.37b 0.93a 0.7b

2.125 (aerobic tank) 75–110

107–740

Operational parameters DO in aeration basin (mg L−1 )

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SVI (ml g−1 )

93a 116b

a Average value during dry season. b Average value during rainy season.

HRT (1.125 days) and RAS rate of approximately 150%. The pre-anoxic denitrification, complete mix, extended aeration AS system operated under complete solids retention, high RAS rate (600% at steady state conditions), highly aerobic conditions in the aeration basin (DO > 4 mg L−1 ) and under high HRT (1.25 and 2.25 days in pre-anoxic and aerobic tanks, respectively). The quality of the effluent remained in accordance with the demands of the enforced legislation (Table 1). The preliminary system achieved average COD and TN reduction of 32%. In more detail, under low DO concentration conditions, low COD removal and low nitrification rates result, as the AS flocs will be partially aerobic. Denitrification occurs in the anoxic zones within the floc particles due to oxygen depletion.[32] The high performance of the pre-anoxic denitrification, complete mix, extended aeration AS system (average COD and TN removal efficiencies 95% and 85%, respectively) is achieved mainly due to the high biomass concentration achieved in steady-state operating conditions and by complete SRT, high HRT and high DO concentration.[16,33] Furthermore, because of high DO concentrations, relative small flocs were produced with a higher compactness, which assisted in obtaining relatively low SVI values.[34] The entire WWTP reached overall COD and TN removal efficiencies of 98% and 90%, respectively. Generally, three phases can be identified from the beginning to the stabilization of an AS system. Based on the observed changes in sludge microbiological and morphological characteristics, in WWTP operational conditions and in accordance with other studies, [10,35] the studied extended aeration process was separated into three distinguished periods. In these periods, the WWTP operational parameters of MLSS, MLVSS, F/M and SUR (Figure 1) were linked to specific AS morphological characteristics and to the presence of indicative microbial

species (Figures 2–4). These periods can be described as: (a) start-up and high-growth phase, where microorganisms indicative of young AS are observed, (b) low-growth phase, with mature biomass species and good floc aggregation, (c) stabilization phase, with very low-growth rate, where old biomass microbial species and by pin point flocs are observed. The duration of each phase depends on various parameters, such as organic load, SRT and wastewater type and significantly varies between different WWTPs. In these periods, F/M and SUR followed a corresponding rapid decrease, less rapid decrease and plateau phase in which minimum mean values were obtained. Because of the very high COD removal efficiency, the F/M and SUR minimum values became equal and their curves coincided in plateau phase. The evolution of F/M, SUR, MLSS and MLVSS is presented in Figure 1. In the first period, which includes the start-up and the high-growth phase, excess of substrate resulted in high-growth rates because growth dominated against the endogenous processes.[36] The initial microbial species encountered in the start-up phase, which are usually present in typical raw sewage, such as free-swimming bacterivorous ciliates and small heterotrophic flagellates, cannot be considered typical of WWTP’s environment because they are not linked to the presence of sludge flocs.[3] With the growth of sludge, these non-indicative species are replaced by other more functional groups of microorganisms. In this first period, which in this study lasted for 90 days, the F/M ratio obtained values near the higher corresponding ratios for extended aeration processes (F/M > 0.105 kgBOD5 kgMLVSS−1 d−1 ). The AS in this period is referred to as young sludge and is characterized by SRT lower than 90 days, high F/M ratios, high SUR values and rapid bacterial growth (Figure 2). Additionally, AS is characterized by relatively large floc size, low SVI (SVI = 75–95 ml g−1 ) as well as by the presence

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Figure 1. Evolution of MLSS, MLVSS, F/M and SUR in the slaughterhouse WWTP during the three operation periods.

Figure 2. AS high-growth phase (first period) representative microscope photo ( × 400) and MLSS, MLVSS, F/M and SUR evolution in the slaughterhouse WWTP.

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Figure 3. AS low-growth phase (second period) representative microscope photos ( × 400) and MLSS, MLVSS, F/M and SUR evolution in the slaughterhouse WWTP.

Figure 4. AS stabilization phase (third period) representative microscope photos ( × 100, × 400) and MLSS, MLVSS, F/M and SUR evolution in the slaughterhouse WWTP.

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of amoebas, flagellates, free swimming and crawling ciliates. Amoebas and flagellates usually are present during the start-up of a WWTP, while flagellates prefer soluble nutrients and dead or decaying material. From day 30 (F/M = 0.2 kgBOD5 kgMLVSS−1 d−1 ), ciliates predominated over the other species, because of the lower soluble organic nutrients’ availability than that measured in the start-up of the treatment process (Figure 2). In the second period, which includes the low-growth phase, a great variety of microbial species were present in the sludge, referred to as mature sludge (Figure 3). In this period, which lasted from day 90 to day 180, the F/M ratio decreased to values approximating the minimum corresponding values of extended aeration processes (0.105–0.06 kgBOD5 kgMLVSS−1 d−1 ). Mature sludge is characterized by SRT between 90 and 180 days, lower F/M ratios, lower SUR values and lower bacterial growth rates than of those observed in the first period (Figure 1). In this low-growth phase (Figure 3), as the amount of available organic substrate decreased and the DO was available more, protozoa population became more complex and thus free-swimming ciliates, crawling ciliates, stalk ciliates and metazoan (rotifers) were more commonly encountered.[3] The flocs were relatively small with good settling characteristics but with higher SVI values (SVI = 85–105 ml g−1 ) than in the first period. The presence of ciliates, which feed on bacteria not on dissolved organics, indicated goodquality sludge that is typically found in young to medium age sludge.[37] Finally, the third period, which includes the negligible growth phase, and lasts from the 180th day to the end of the study (425th day), is considered as the stabilization phase. During this phase, the AS microbiological and morphological characteristics remained relatively constant, as well as the MLSS and MLVSS concentration, the F/M ratio and the SUR rate (Figure 1). In this period the AS is referred to as old sludge and is characterized by SRT > 180 days, extended starvation conditions which result in F/M approximating minimum extended aeration values (F/M = 0.055 kgBOD5 kgMLVSS−1 d−1 ), low SUR rates, pin point flocs and increased presence of predator species such as rotifers (Figure 4). Despite the increased MLSS concentrations resulting in high solids loading rates and despite the pin point floc formation resulting in higher SVI values (SVI = 95–120 ml g−1 ), efficient sedimentation and therefore good effluent quality was achieved through forced sedimentation imposed in the clarifiers by an RAS rate over 600%.[15] Free-swimming ciliates, crawling ciliates, colonial stalked ciliates and increased presence of rotifers describe the biological characteristics of activated sludge during this phase of WWTP operation (Figure 4). The observed stalked ciliates indicated good floc formation, sufficient DO and efficient organic compounds removal, while increased presence of rotifers indicated that the AS process had reached the stabilization

phase.[9,13] Furthermore, the presence of rotifers in AS contributed to the removal of effluent turbidity by consuming non-flocculated bacteria and to lowering the BOD and TSS concentrations in the effluent.[25] Additionally, the structure of the ciliate community in the stabilization phase reflected the stable condition of the aeration tank environment, with a balance between the organic loading and the sludge that was produced, removed and recycled.[3] It is noteworthy that filamentous and nematode species were not observed during the 425 days of operation. Filamentous bacteria, which have a higher surface-tovolume ratio than that of their floc-forming counterparts, are able to predominate under low DO, high SRT, low F/M, low nutrient conditions or high sulphide levels (environmental leverage). Consequently, in this WWTP, the higher than 4 mg L−1 DO concentration in the extended aeration tank and the sufficient nutrient content prevented the growth of filamentous microorganisms, such as Microthrix parvicella and Type 0041.[38–41] Additionally, in this high substrate concentration process (average influent COD 4150 mg L−1 ), the preliminary simultaneous nitrification/denitrification, which can be described as an anoxic/aerobic selector, reduces the readily biodegradable substrate (rbCOD) that favours filamentous growth.[2] The contact time in the preliminary treatment (HRT = 0.75 days) was enough to achieve a COD reduction of 32% and thus the remaining substrate (average COD 2835 mg L−1 ) did not favour the growth of the so-called low F/M filamentous bacteria group in the complete mix extended aeration compartment, such as morphotypes M. parvicella and Types 021N, 0041, 0675, 0092 and 0581.[22] The equalization tank installed prior to the preliminary treatment diminished temperature, flow and load fluctuations, improving the operation of the anoxic/aerobic selector. Nematodes are crawling species that are usually encountered in high SRT and high DO wastewater treatment processes. In the AS process studied, they are present in relatively small numbers because the pin point flocs of AS at the stabilization phase are not suitable for crawling, which they prefer over the free-swimming mode. In trickling filters where the fine stationary substratum is suitable to permit crawling, nematodes are quite abundant.[42] Fruit canning industry complete solids retention AS WWTP The fruit canning industry had worked for three months, due to the seasonal operation of the industry and therefore the WWTP was monitored only for 87 days since startup. The operational parameters of the treatment facility are presented in Table 2 while the evolution of F/M, SUR, MLSS and MLVSS is shown in Figure 5. The WWTP operates under highly aerobic conditions (DO > 4 mg L−1 ), with complete solids retention, relatively high HRT (1.4 days) and high RAS rate (300%).

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Figure 5. Evolution of MLSS, MLVSS, F/M, SUR and SVI in the fruit canning industry’s WWTP, during the two operation periods.

Generally, the fruit canning wastewaters composition is characterized by nitrogen deficiency and limited phosphorus content.[43] The influent wastewater composition, which is presented in Table 1, results in an average influent COD:N:P ratio of 150:0.33:0.2, indicating mainly nitrogen deficiency. It is well known that bacteria cells consist of approximately 12% w/w nitrogen and 6–7% w/w phosphorus; thus nitrogen and phosphorus compounds are essential for bacterial growth and can become the growth-limiting factor in AS processes.[36,37] Based on the observed changes in biomass growth rates, in AS microbiological and morphological characteristics as well as in substrate availability, two distinguished periods were observed in the treatment process: (a) the first period, which includes the lag phase and (b) the second period, which includes the treatment start-up and the high-growth phase. The low-growth phase and stabilization phase were not observed in this complete solids retention process due to the limited operation time (87 days). The first period (lag phase), which lasted for approximately 20 days, is characterized by nutrient deficiency and limited biomass growth. Furthermore, filamentous bacteria (Type 021N and Thiothrix I ) predominated over floc-forming bacteria during the first month of WWTP operation, resulting in voluminous, non-cohesive flocs (SVI = 400–730 ml g−1 ) causing insufficient sedimentation and therefore bad effluent quality (Figure 6). The nitrogen-deficient environment (COD:N:P 150:0.33:0.2) is the reason behind the delay in biomass growth and the increased presence of unwanted filamentous species. Therefore, the periodic addition of urea-nitrogen from the

20th day onwards ensured a minimum COD:N:P ratio of 150:3.7:0.2 and an average of 150:6.1:0.2, which proved to be sufficient for biomass growth and relatively good sludge characteristics. The second period is characterized by a high biomass growth triggered by external nitrogen addition in the system on the 20th day (Figure 7). In this period, which includes the process start-up and the high-growth phase, decrease of filamentous microorganisms and efficient wastewater treatment were observed. The flocforming bacteria (Figure 7) dominated over the filamentous (Figure 6) and when a critical concentration was reached, more cohesive, less voluminous and more durable flocs (Figure 7), easier to settle, were created (SVI 95–120 ml g−1 ). Towards the end of the WWTP operation and until the 87th day, a low tendency to MLVSS growth stabilization appeared and limited growth of predator species was observed. Consequently, the high MLVSS growth in this period and the delayed appearance of metazoan species are attributed to the lag phase (first period) and the high F/M ratios (0.4–0.25 kgBOD5 kgMLVSS−1 d−1 ), indicating that the WWTP did not reach low-growth and stabilization phases.

Municipal WWTP The wastewater of Paralimni municipality integrates an anaerobic selector for phosphorus removal, a pre-anoxic denitrification compartment and an extended aeration/ nitrification basin. The WWTP’s operational parameters during dry and rainy periods are presented in Table 2. It is worth mentioning that MLSS, MLVSS and sludge

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Figure 6. AS lag phase (first period) representative microscope photos ( × 100) and MLSS, MLVSS, F/M, SUR and SVI evolution in the fruit canning industry’s WWTP.

Figure 7. AS high-growth phase (second period) representative microscope photos ( × 100) and MLSS, MLVSS, F/M, SUR and SVI evolution in the fruit canning industry’s WWTP.

waste remained relatively constant during the three distinguished monitoring periods. The WWTP, during the two dry periods, operated at HRT and SRT of approximately 1 and 13 days, respectively, RAS rate between 50% and 100%, DO content and conductivity in aeration basin of 3.3–4.9 mg L−1 and 2230–3190 µS cm−1 , respectively,

as well as at average F/M ratio of approximately 0.29 kgBOD5 kgMLVSS−1 d−1 (Figure 8). During the rainy period, lower average HRT of 0.7 days, higher average F/M ratios of approximately 0.38 kgBOD5 kgMLVSS−1 d−1 , higher DO concentration of 3.8–7.6 mg L−1 and lower conductivity of 1930–2630 µS cm−1 in aeration basin

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Figure 8. Evolution of influent conductivity, F/M and SVI in the municipal WWTP during dry and rainy periods.

were recorded (Figure 8). During the rainy period, the lower HRT, the higher F/M and the lower nutrient content are attributed to the increase of volumetric load caused by the inflow of rainwater in the sewage system and due to the lack of a stabilization tank. Furthermore, the increase of DO is attributed to the decrease of conductivity due to dilution with rainwater. The variations of F/M, conductivity, DO and SVI (Figure 8), during the three monitoring periods (dry-rainy-dry) were correlated with the observed changes of AS’s biological and morphological characteristics and the appearance of filamentous bacteria. Filamentous bacteria increased sludge volume, reduced floc aggregation and compromised WWTP’s efficient treatment. In the first monitoring period (dry period), relative stable sludge characteristics and properties as well as good settling efficiency were observed. The average wastewater COD:N:P ratio of approximately 150:5.86:1.5 was adequate for sufficient biomass growth and efficient biological treatment. A few Type 0041 and/or Type 0675 filamentous bacteria were present engulfed by AS flocs and SVI values remained relatively low and almost constant (87– 92 ml g−1 ). It is worth mentioning that a small number of filamentous bacteria inside the AS floc improve the cohesiveness of the sludge, thus ensuring good sedimentation efficiency and effluent quality. During the second monitoring period (rainy period), the six-day rainfall led to changes in influent wastewater composition and WWTP operation conditions (Tables 1 and 2).

Changes in AS biological and morphological characteristics were observed. Excessive growth of filamentous bacteria resulted in a more voluminous AS with SVI 91–136 ml g−1 (Figure 8). In order to restrain the sludge bulking phenomena that deteriorated sedimentation efficiency and effluent quality, identification of the filamentous species and correlation with the second period’s operating conditions were performed. In this period, characterized by highly aerobic conditions, the COD:N:P ratio is the determining factor for filamentous growth. The microscopic examination of the AS revealed that Type 021N was the predominant filament responsible for sludge bulking during the first three rainy days (sixth to eighth day). During these three days, SVI ranged between 91 and 109 ml g−1 , average F/M ratio was 0.35 kgBOD5 kgMLVSS−1 d−1 and average COD:N:P ratio was 150:4.05:1.06, lower than the minimum required nutrient content. During the next rainy days (9th to 12th day), SVI ranged between 121 and 136 ml g−1 , average F/M increased at 0.40 kgBOD5 kgMLVSS−1 d−1 and average COD:N:P further decreased to 150:3.37:0.52. At that time, Thiothrix I and II predominated over Type 021N filamentous bacteria. It is well known that in nitrogen- and phosphorus-scarce environment Type 021N and Thiothrix I, II filamentous bacteria thrive. Both of them are strictly aerobic and can grow in a wide range of F/M values (0.05–0.4 kgBOD5 kgMLVSS−1 d−1 ) with greater chances for massive growth at F/M over 0.1 kgBOD5 kgMLVSS−1 d−1 .[44] The rapid growth of Thiothrix I and II and the proliferation over Type

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Figure 9. Flowchart of WWTP processes control–Logical steps towards the best solution of WWTP operating problems.

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Environmental Technology 021N implies that although both species thrive in nutrientdeficient conditions, Thiothrix I and II prevail in COD:N:P ratios lower than 150:4:1, in F/M ratios higher than 0.35 kgBOD5 kgMLVSS−1 d−1 and in DO higher than 6.9 mg L−1 (Figure 8). In the third monitoring period (second dry period), the sedimentation efficiency improved, resulting in better effluent quality. Decrease of filamentous bacteria and recovery to steady WWTP operating conditions, similar to those of the first period, were observed. During the third period, SVI reached an average value of 95 ml g−1 , F/M and COD:N:P ratio obtained average values of 0.29 kgBOD5 kgMLVSS−1 d−1 and of 150:5.61:1.39, respectively, typical of WWTP steady-state operating conditions (Figure 8). In this study, the most common microbe-related problems and symptoms in high load AS processes as well as the proposed solutions are presented in a logic flowchart (Figure 9). Symptoms such as turbid effluent, sludge foaming and sludge bulking, which are often caused due to the presence or absence of specific microbial species, are correlated with operating conditions. A practical way for plant performance controlling is presented in a logic flowchart (Figure 9) in which operational parameters are linked to microbial manipulation, resulting in a useful tool for researchers and engineers.

Conclusions The growth of AS, in nutrient-sufficient AS aerobic processes, can be distinguished into three phases, whose duration depends on SRT and F/M: (a) the start-up and high-growth phase (young sludge), (b) the less rapid growth phase (mature sludge) and (c) the stabilization phase (old sludge, minimized sludge yields). Representative microbial species and specific floc morphology describe each operational phase. SRT also affects the nutrient demand of each phase. In high SRT AS processes, the low observed yields, indicating starvation conditions, result in lower nutrient demand, preventing filamentrelated problems caused by relatively low COD:N:P ratios. In this study, in a complete solids retention, highly aerobic (DO > 4 mg L−1 ) AS WWTP, a minimum COD:N:P ratio of 150:3.7:0.2 proved to be sufficient for biomass growth and good sludge characteristics. On the contrary, in a 13day SRT, highly aerobic (DO > 3.3 mg L−1 ) AS WWTP, this COD:N:P ratio proved to be insufficient as Type 021N and Thiothrix I,II filamentous bacteria grew. Furthermore, lowering F/M and SUR values by reducing the influent’s readily biodegradable compounds in a preliminary simultaneous nitrification/denitrification process inhibits filamentous bacteria growth. The WWTP’s operational parameters, problems and symptoms and microbial manipulation can be linked in the form of a logic flowchart for controlling plant performance.

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Disclosure statement No potential conflict of interest was reported by the authors.

References [1] Kaewpipat K, Grady CP Jr. Microbial population dynamics in laboratory-scale activated sludge reactors. Water Sci Technol. 2002;46:19–27. [2] Henze M, van Loosdrecht MCM, Ekama GA, Brdjanovic D. Biological wastewater treatment: principles, modelling and design. IWA; 2008. [3] Madoni P. Protozoa in wastewater treatment processes: A minireview. Ital J Zool. 2011;78:3–11. [4] Madoni P. Quantitative importance of ciliated protozoa in activated sludge and biofilm. Bioresource Technol. 1994;48:245–249. [5] Cordi L, Assalin MR, Ponezi AN, Durán N. Identification of microbiota for activated sludge acclimated by paper mill effluent Kraft E1, bioremediation. J Bioremed Biodeg. 2012;3:169. [6] Al-Shahwani SM, Horan NJ. The use of protozoa to indicate changes in the performance of activated sludge plants. Water Res. 1991;6:633–638. [7] Ratsak CH, Maarsen KA, Kooijman SAL. Effects of protozoa on carbon mineralization in activated sludge. Water Res. 1996;30:1–12. [8] Nicolau A, Dias N, Mota M, Lima N. Trends in the use of protozoa in the assessment of wastewater treatment. Res Microbiol. 2001;152:621–630. [9] Akpor OB, Muchie M. Bioremediation of polluted wastewater influent: phosphorus and nitrogen removal. Sci Res Essays. 2010;5:3222–3230. [10] Madoni P. Growth and succession of ciliate populations during the establishment of a mature activated sludge. Acta Hydrobiologica. 1982;24:223–232. [11] Curds CR, Cockburn A. Protozoa in biological sewage treatment processes – II. Protozoa as indicators in the activated sludge process. Water Res. 1970;4:237–249. [12] Amann R, Lemmer H, Wagner MH. Monitoring the community structure of wastewater treatment plants: a comparison of old and new techniques. FEMS Microbiol Ecol. 1998;25:205–215. [13] Lee Natuscka M, Welander T. Reducing sludge production in aerobic wastewater treatment through manipulation of the ecosystem. Water Res. 1996;30:1781–1790. [14] Wei Y, van Houten RT, Borger AR, Eikelboom DH, Fan Y. Minimization of excess sludge production for biological wastewater treatment. Water Res. 2003;37: 4453–4467. [15] Amanatidou E, Samiotis G, Trikilidou E, Pekridis G, Taousanidis N. Evaluating sedimentation problems in activated sludge treatment plants operating at complete sludge retention time. Water Res. 2015;69:20–29. [16] Amanatidou E, Samiotis G, Bellos D, Pekridis G, Trikiloidou E. Net biomass production under complete solids retention in high organic load activated sludge process. Bioresource Technol. 2015;182:193–199. [17] Liu Y, Tay JH. State of the art of biogranulation technology for wastewater treatment. Biotechnol Adv. 2004;22: 533–563. [18] Poole JEP. A study of the relationship between the mixed liquor fauna and plant performance for a variety of effluent quality in activated sludge sewage treatment works. Water Res. 1984;18:281–287.

Downloaded by [University of Connecticut] at 00:15 04 October 2017

278

E. Amanatidou et al.

[19] Orhon D, Artan N. Modeling of activated sludge systems. Lancaster: Technomic; 1994. [20] Liu Y, Fang HHP. Influences of extracellular polymeric substances (EPS) on flocculation, settling, and dewatering of activated sludge. Crit Rev Environ Sci Technol. 2003;33:237–273. [21] Abbassi B, Dullstein S, Rabiger N. Minimization of excess sludge productio by increase of oxygen concentration in activated sludge flocs; experimental and theoretical approach. Water Res. 1999;34:139–146. [22] Jenkins D, Richard M, Daigger GT. Manual on the causes and control of activated sludge bulking, foaming, and other solids separation problems. 3rd ed. Boca Raton, FL: CRC Press; 2004. [23] Martins AMP, Heijnen JJ, van Loosdrecht MCM. Effect of dissolved oxygen concentration on sludge settleability. Appl Microbiol Biotechnol. 2003;62:586–593. [24] Corbitt RA. Standard handbook of environmental engineering. 2nd ed. New York (NY): McGraw-Hill; 1998. [25] Loosdrecht M, Henze M. Maintenance, endogenous respiration, lysis, decay and predation. Water Sci Technol. 1999;39:107–117. [26] Stephenson T, Judd S, Jeferson B, Brindle K. Membrane bioreactors for wastewater treatment. 1st ed. London: IWA; 2000. [27] Pollice A, Laera G, Blonda M. Biomass growth and activity in a membrane bioreactor with complete sludge retention. Water Res. 2004;38:1799–1808. [28] Laera G, Pollice A, Blonda M. Zero net growth in a membrane bioreactor with complete sludge retention. Water Res. 2005;39:5241–5249. [29] Jarvis P, Jefferson B, Gregory J, Parsons SA. A review of floc strength and breakage. Water Res. 2005;39: 3121–3137. [30] Massé DI, Masse L. Characterization of wastewater from hog slaughterhouses in Eastern Canada and evaluation of their in-plant wastewater treatment systems. Can Agric Eng. 2000;42:139–146. [31] Pozo R, Tas DO, Dulkadiroglu H, Orhon D, Diez V. Biodegradability of slaughterhouse wastewater with high blood content under anaerobic and aerobic conditions. J Chem Technol Biotechnol. 2003;78:384–391.

[32] Holman JB, Wareham DG. COD, ammonia and dissolved oxygen time profiles in the simultaneous nitrification/denitrification process. Biochem Eng J. 2005;22: 125–133. [33] Lubbeke S, Vogelpohl A, Dewjanin W. Wastewater treatment in a biological high-performance system with high biomass concentration. Water Res. 1995;29:793–802. [34] Wilen BM, Balmer P. The efect of dissolved oxygen concentration on the structure, size and size distribution of activated sludge flocs. Water Res. 1999;33:391–400. [35] Madoni P, Antonietti R. Colonization dynamics of ciliated protozoa populations in an activated sludge plant. In Proceeding IV Italian symposium of population dynamics parma; 1984. p. 105–112. [36] Metcalf and Eddy (revised by: Tchobanoglous G., Burton L.F., Stensel H.D.). Boston (MA): Wastewater Engineering: Treatment and Reuse, McGraw Hill; 2003. [37] Gerardi HM. Wastewater bacteria. Hoboken: Wiley; 2006. [38] Gaval G, Pernell JJ. Impact of the repetition of oxygen deficiencies on the filamentous bacteria proliferation in activated sludge. Water Res. 2003;37:1991–2000. [39] Martins AMP, Pagilla K, Heijnen JJ, van Loosdrecht MCM. Filamentous bulking sludge – a critical review. Water Res. 2004;38:793–817. [40] Rossetti S, Tomei MC, Nielsen PH, Tandoi V. “Microthrix parvicella”, a filamentous bacterium causing bulking and foaming in activated sludge systems: a review of current knowledge. FEMS Microbiol Rev. 2005;29:49–64. [41] Tian WD, Li WG, Zhang H, Kang XR, van Loosdrecht MCM. Limited filamentous bulking in order to enhance integrated nutrient removal and effluent quality. Water Res. 2011;45:4877–4884. [42] Spellman FR. Handbook of water and wastewater treatment plant operations. 3rd ed. Boca Raton: CRC Press; 2000. [43] Diamantis ID, Vaiopoulou E, Aivasidis A. Fundamentals and applications of anaerobic digestion for sustainable treatment of food industry wastewater. In: Oreopoulou V, Russ W, editor. Utilization of By-Products and Treatment of Waste in the Food Industry. New York: Springer, Vol. 3; 2007. p. 73–97. [44] Eikelboom DH. Process control of activated sludge plants by microscopic investigation. London: IWA; 2000.

Influence of wastewater treatment plants' operational conditions on activated sludge microbiological and morphological characteristics.

The effect of wastewater composition and operating conditions in activated sludge (AS) microbiological and morphological characteristics was studied i...
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