Solids Retention Time Research Articles

Digesta passage and nutrient digestibility in Boran steers at low feed intake levels.

The present study evaluated the effects of energetic undernutrition on liquid and solid digesta passage and on nutrient digestibility as well as their interdependencies. Using a 4x4 Latin square design, 12 growing Boran steers (183±15.2kg live weight) were allocated to four levels of metabolizable energy (ME) supply fixed at 100, 80, 60 and 40% of individual maintenance energy requirements (MER) during four experimental periods. Each period comprised three weeks of adaptation, two weeks of data collection and two weeks of recovery. Diets MER80, MER60 and MER40 only consisted of Rhodes grass hay (RGH), whereas diet MER100 contained (as fed) 83% RGH, 8% cotton seed meal and 9% sugarcane molasses. Feed intake differed between treatments (p<.001) and ranged from 40±0.6g dry matter (DM) per kg of metabolic weight (kg0.75 ) in MER40 to 81±1.3g DM in MER100. Digestibility of neutral and acid detergent fibre (NDF, ADF) was highest at MER80, whereas rumen retention time of liquid and solid digesta was longest at MER40. The correlation of rumen retention time of liquid and solid digesta with the digestibility of proximate diet components was weak but positive, whereas the correlation of liquid and solid rumen retention time with quantitative feed and nutrient intake was strong (p<.01) and negative. Our results suggest that tropical cattle are able to buffer a moderate energy deficit by prolonging rumen retention time of digesta and hence improve diet digestibility. Conversely, a severe energy deficit cannot be buffered by digestive adaptation mechanisms and will inevitably lead to productivity losses.

Impact of solids retention time on the biological performance of an AnMBR treating lipid-rich synthetic dairy wastewater

ABSTRACT In this study, the impact of applied solids retention time (SRT) on the biological performance of an anaerobic membrane bioreactor (AnMBR) treating synthetic dairy wastewater with high lipid content was assessed. Two side-stream AnMBR systems were operated at an SRT of 20 and 40 days (R20 and R40, respectively), equipped with an inside-out tubular membrane operated in cross-flow mode under full-scale operational conditions, i.e. crossflow velocity, transmembrane pressure, membrane flux. Successful operation was achieved and removal efficiencies of both reactors were up to 99% applying an organic loading rate (OLR) of 4.7 g COD L−1 d−1. No precipitation of lipids was observed throughout the operational period, keeping the lipids available for the anaerobic degradation. Long chain fatty acid (LCFA) accumulation was very modest and amounted 148 and 115 mg LCFA-COD per gram of volatile suspended solids (VSS) for R20 and R40, respectively. At an SRT of 40 days, a slightly better biological conversion was obtained. Periodically performed specific methanogenic activity (SMA) tests showed stabilization of the SMA for R40 sludge, whereas for R20 sludge the SMA continued to decrease. This study revealed a more stable reactor performance operating the AnMBR at an SRT of 40 days compared to 20 days.

Acclimatised rumen culture for raw microalgae conversion into biogas: Linking microbial community structure and operational parameters in anaerobic membrane bioreactors (AnMBR)

Ruminal fluid was inoculated in an Anaerobic Membrane Reactor (AnMBR) to produce biogas from raw Scenedesmus. This work explores the microbial ecology of the system during stable operation at different solids retention times (SRT). The 16S rRNA amplicon analysis revealed that the acclimatised community was mainly composed of Anaerolineaceae, Spirochaetaceae, Lentimicrobiaceae and Cloacimonetes fermentative and hydrolytic members. During the highest biodegradability achieved in the AnMBR (62%) the dominant microorganisms were Fervidobacterium and Methanosaeta. Different microbial community clusters were observed at different SRT conditions. Interestingly, syntrophic bacteria Gelria and Smithella were enhanced after increasing 2-fold the organic loading rate, suggesting their importance in continuous systems producing biogas from raw microalgae.

Importance of Species Sorting and Immigration on the Bacterial Assembly of Different-Sized Aggregates in a Full-Scale Aerobic Granular Sludge Plant.

In aerobic granular sludge (AGS) systems, different-sized microbial aggregates having different solids retention time (SRT) coexist in the same reactor compartment and are subjected to the same influent wastewater. Thus, the AGS system provides a unique ecosystem to study the importance of local (species sorting) and regional (immigration) processes in bacterial community assembly. The microbial communities of different-sized aggregates (flocs <0.2 mm, small granules (0.2-1.0 mm) and large granules >1.0 mm), influent wastewater, excess sludge and effluent of a full-scale AGS plant were characterized over a steady-state operation period of 6 months. Amplicon sequencing was integrated with mass balance to determine the SRT and net growth rate of operational taxonomic units (OTUs). We found strong evidence of species sorting as opposed to immigration, which was significantly higher at short SRT (i.e., flocs and small granules) than that at long SRT (large granules). Rare OTUs in wastewater belonging to putative functional groups responsible for nitrogen and phosphorus removal were progressively enriched with an increase in microbial aggregates size. In contrast, fecal- and sewage infrastructure-derived microbes progressively decreased in relative abundance with increase in microbial aggregate size. These findings highlight the importance of AGS as a unique model ecosystem to study fundamental microbial ecology concepts.

Effect of solid–liquid separation enhanced by low-temperature hydrolysis in methanogenic phase on two-phase anaerobic sludge digestion system

In this study, a mesophilic two-phase anaerobic digestion system was used to treat waste activated sludge at the temperature of 35 ± 1 °C. The internal structure of the anaerobic sequencing batch reactor (methanogenic phase) has been improved by providing a solid–liquid separation plate, and a continuous-flow stirred tank reactor was employed as the acidogenic phase. The results of the study indicate that solid–liquid separation can be achieved in the methanogenic phase, and the low-temperature hydrolysis pretreatment can enhance solid–liquid separation capability. The separation of solids retention time and hydraulic retention time was also achieved in methanogenic phase, and the solids retention time was extended, which effectively improved the anaerobic digestion performance of the system. The organic concentration and total solids concentration in the methanogenic reactor were significantly different in height. The bottom of the reactor formed an area of high solids and low organic precipitation, while the upper part of the reactor formed a clear water area. The suspended solids concentration in the reactor clear water area was maintained at 359 ± 84 to 533 ± 93 mg/L, and the better solid–liquid separation effect was obtained inside the methanogenic phase. The maximum biogas yield was 258 ± 3 mL/g with the maximum volatile solids degradation rates of 59.43% in the system in Stage 3. Considering the efficiency of anaerobic digestion and the operating costs, the operating condition of Stage 4 (the designed hydraulic retention time was 10.67 d, the feed sludge was thermally pretreated, and its volume was 3 L) in this test is the optimal.

Fate of sterols, polycyclic aromatic hydrocarbons, pharmaceuticals, ammonia and solids in single-stage anaerobic and sequential anaerobic/aerobic/anoxic sludge digestion

Emerging contaminants (ECs), such as pharmaceuticals, sterols and polycyclic aromatic hydrocarbons (PAHs) are frequently detected in the environment. ECs are refractory, toxic, tend to bioaccumulate and have a potential to disrupt the endocrine system of living organisms. These compounds are only partially eliminated in wastewater treatment plants (WWTPs). Due to their hydrophobic nature, they tend to accumulate in sludge. However, the fate of the majority of ECs in sludge treatment processes is not fully understood. In this study, the effect of a sequential anaerobic/aerobic/anoxic (AN/AERO/ANOX) digestion and a conventional single-stage AN digestion (as control) was investigated on mixed primary and secondary sludge. Digesters were operated at an overall solid retention time (SRT) of 18 days. The steady-state results have shown that sequential AN/AERO/ANOX digestion configurations improved the removal of three classes of ECs (e.g., sterols, PAHs and pharmaceuticals) by either reducing their accumulation or enhancing their removal. Moreover, sequential AN/AERO/ANOX digestion also achieved 45% less ammonia generation, 20% faster digestate dewaterability and 4% enhanced volatile solids removal compared to single-stage AN digestion.

An overview of technologies to recover phosphorus as struvite from wastewater: advantages and shortcomings.

Phosphorus (P) is a significant limiting nutrient which is essential for all forms of lives. However, phosphate rock reserves are depleting rapidly due to population growth. At the same time, several countries have imposed legislative regulations on P-release into surface waters due to eutrophication. Nutrient recovery from wastewater can facilitate a sustainable, cost-effective and environment-friendly source of phosphorus. Although P-recovery as struvite from wastewater has been widely studied for a long time, there still exists a lot of challenges for widespread full-scale implementation. This paper presents a comprehensive analysis of the current state of the technologies for phosphorus recovery in the form of struvite. Fluidized bed reactors (FBRs) are widely used compared to continuously stirred reactors for P-recovery as struvite because of different solid and liquid retention time. Commercially available technologies were reported to accomplish about 80% P-removal efficiencies with a reasonable P-recovery for themost of the cases. The struvite production rate of various technologies varies from 0.89 to 13.7kg/kg influent P. Nevertheless, these technologies are associated with several shortcomings such as high operational costs, high energy consumption, and large footprint. Increasing efforts focusing on the development of sustainable and commercially feasible technologies are expected in this sector as P-recovery is considered to be the future of wastewater engineering.

Biological Treatment of 3,6-Dichloro-2-Methoxybenzoic Acid Using Anaerobic-Aerobic Sequential Batch Reactor

A sequential anaerobic-aerobic batch reactor was used to treat 3,6-dichloro-2-methoxybenzoic acid (dicamba) during a long operation period of 340 days in the presence of disodium anthraquinone-2,6-disulphonate (AQS) as redox mediator. The sludge activity was evaluated for different dosages of dicamba over constant hydraulic retention time (HRT), neutral pH (6.5–7.5) and at ambient reactor temperature. Effects of increased dicamba concentration, solids retention time (SRT) and oxidation reduction potential (ORP) on the biodegradation of dicamba was monitored and compared with control reactor containing no dicamba. Results revealed that long operation period, long SRT and ORP were playing important role in the breakdown of dicamba to its transformation products and subsequent removal in the system. The system was capable of degrading the compound completely during long operation period, long SRT and at low ORP in the presence of AQS. Reducing condition in the anaerobic reactor significantly contributed to the treatment process through demethylation, dehalogenation and dechlorination reactions in the presence of different reducing bacteria. The results of GC-HRMS identified the anaerobic transformation products of dicamba as oleic acid (C18H34O2), 9-Octadecenoic acid (Z), 2-hydroxy-1-(hydroxymethyl)ethyl ester (C21H40O4), trans-13-Ocatadecenoic acid (C18H34O2) compounds which were then oxidised in the aerobic reactor.

Factors affecting SVI in small scale WWTPs

This paper analyses factors associated with bulking in 195 small scale wastewater treatment plants (WWTPs) in Estonia. Operational data from each plant were collected and analysed statistically. The key factors associated with bulking were infiltration into sewage pipes, the type and purpose of process reactor, operational practices and influent characteristics. Both anaerobic fraction and volumetric fraction of the anaerobic reactor compared to the aerobic reactor resulted in a positive correlation with sludge volume index (SVI) <150 ml/g values. Good operation and maintenance practice as well as an operator's competence play a crucial role in bulking prevention. Using the 30 minute settling test (V30) as the single process control parameter can mislead an operator's judgement in process control strategies and cause effluent violations. Misjudgements in process control decisions can lead to unwanted conditions in small WWTPs (e.g. excessive chemical addition favoured bulking). Use of instrumentation, control and automation helped to keep the process conditions more stable and reduce the probability of bulking. Analyses of variance showed that the factors associated with Microthrix parvicella growth were long solids retention time (SRT), low food-to-microorganism ratio (F/M) and lack of carbon content compared against nitrogen and phosphorus contents.

Microbial population dynamics in temperature‐phased anaerobic digestion of municipal wastewater sludge

AbstractBACKGROUNDThe present study compared the performance and capability of thermophilic–mesophilic, temperature‐phased anaerobic digestion (TPAD) of wastewater sludge at 45 and 55 °C in terms of rate of hydrolysis and methanogenesis at two point interval times – December and January – with operational conditions kept constant (total solid content 6.5%, solids retention time 12.5 days).RESULTSThe reduction of volatile solids (VS) was 77% at 45 °C TPAD‐1 and 72% at 55 °C TPAD‐II. The accumulation of ammonia and volatile fatty acids (VFAs; propionic acid) were observed to be below the inhibitory range (&gt;3000 mg L−1), whereas the methane (CH4) production (45 °C, 3.55 ± 0.47 L CH4 L−1 day−1; 35 °C, 1.44 ± 0.12 L CH4 L−1 day−1) remained considerably higher in TPAD‐I than TPAD‐II. Furthermore, the TPAD‐II system suffered from certain degree of instability due to high level of total VFAs (6087 ± 1578 mg L−1), low buffering capacity, increased level of total NH3 (2982 ± 219 mg L−1) and free NH3 (246 ± 25 mg L−1), and relatively reduced CH4 production (1.69 ± 0.1 L L−1 day). The bacterial and archaeal population of the TPADs investigated by 454 pyrosequencing and Illumina sequencing showed, in both TPAD systems, a bacterial community dominated by Firmicutes, followed by Bacteriodetes, Proteobacteria, Synergistetes and Actiniobacteria. The archaeal community was dominated by Methanimicrobia (74–84% Methanosarcina) and Methanobacteria (15–27% Methanobacterium). A progression from genus Clostridium to Coprothermobacter and Tepidanaerobacter, and Methanocarcina to Methanothermobacter and Methanobacterium was observed in TPAD‐II.CONCLUSIONSThis study proved the processes driving the dynamics of key microbial population and its correlation with hydrolytic functionality of TPAD systems. © 2019 Society of Chemical Industry

Structural dynamics and transcriptomic analysis of Dehalococcoides mccartyi within a TCE-Dechlorinating community in a completely mixed flow reactor

A trichloroethene (TCE)-dechlorinating community (CANAS) maintained in a completely mixed flow reactor was established from a semi-batch enrichment culture (ANAS) and was monitored for 400 days at a low solids retention time (SRT) under electron acceptor limitation. Around 85% of TCE supplied to CANAS (0.13 mmol d−1) was converted to ethene at a rate of 0.1 mmol d−1, with detection of low production rates of vinyl chloride (6.8 × 10−3 mmol d−1) and cis-dichloroethene (2.3 × 10−3 mmol d−1). Two distinct Dehalococcoides mccartyi strains (ANAS1 and ANAS2) were stably maintained at 6.2 ± 2.8 × 108 cells mL−1 and 5.8 ± 1.2 × 108 cells mL−1, respectively. Electron balance analysis showed 107% electron recovery, in which 6.1% were involved in dechlorination. 16 S rRNA amplicon sequencing revealed a structural regime shift between ANAS and CANAS while maintaining robust TCE dechlorination due to similar relative abundances of D. mccartyi and functional redundancy among each functional guild supporting D. mccartyi activity. D. mccartyi transcriptomic analysis identified the genes encoding for ribosomal RNA and the reductive dehalogenases tceA and vcrA as the most expressed genes in CANAS, while hup and vhu were the most critical hydrogenases utilized by D. mccartyi in the community.

The use of sugar beet pulp stillage for co-digestion with sewage sludge and poultry manure.

The anaerobic mesophilic co-digestion of sugar beet pulp stillage with poultry manure and municipal sewage sludge was investigated in this study. The sugar beet pulp stillage (SBPS) mono-digestion failed owing to an accumulation of volatile fatty acids, leading to a pH value lower than 5.5. A 20% addition of poultry manure to stillage allowed for stable digestion performance despite high volatile fatty acid (total volatile fatty acids) concentrations of 5500-8500 g m-3 with propionic acid being the predominant one and constituting 72%-76% total volatile fatty acids. For this mixture, the maximum methane production of 418 dm3 kgVSfed-1 was achieved when the reactor was operated at a solids retention time of 20 days and an organic loading rate of 4.25 kgVS m-3 d-1. The co-digestion of stillage with 60% municipal sewage sludge gave the average methane yield of around 357 dm3 kgVSfed-1 for all operational conditions applied, however, the methane percentage of biogas (up to 70%) was far greater than the corresponding values obtained for sugar beet pulp stillage-poultry manure co-digestion. Neither ammonia nor volatile fatty acids destabilised the biogas production, and the volatile fatty acid profile showed the dominance of acetic acid (72%-82% total volatile fatty acids) followed by propionic and butyric acids.

Evaluation of predominant factor for shortcut biological nitrogen removal in sequencing batch reactor at ambient temperature.

The shortcut biological nitrogen removal (SBNR) process requires less aeration and external carbon due to the oxidization of ammonia into nitrite and its direct denitrification to nitrogen gas during the biological nitrogen removal process. However, this process produces a poor effluent containing NH4+, since the system has to maintain a high free ammonia (FA, NH3) concentration. To overcome this drawback, in this study, the solid retention time (SRT) and the dissolved oxygen (DO) concentration were controlled to achieve both a high ammonia removal rate and nitrite accumulation in the sequencing batch reactor (SBR) process, which can remove nitrogen from wastewater to the desired concentration and provide high free ammonia inhibition and continuous shock loading. When sufficient DO was supplied, nitrite did not accumulate with a 20-day SRT, but the wash-out of nitrite oxidizers in a shorter SRT resulted in a high nitrite accumulation. When DO acted as a limitation, nitrite accumulated at all SRTs. This indicates that nitrite accumulation is more highly influenced by SRT and DO concentration than by FA inhibition. Also, as nitrite accumulated over a 10-day SRT regardless of DO concentration, the accumulation was more highly influenced by SRT than by DO concentration.

Ammonium removal mechanisms in a microalgal-bacterial sequencing-batch photobioreactor at different solids retention times

Microalgal-bacterial consortia have important advantages over conventional activated sludge systems by achieving full nitrification and organic carbon oxidation without the need of external oxygen supply. This study assessed the different ammonium removal mechanisms and oxygen production of a microalgal-bacterial consortium at different solids retention times (SRT) of 52, 48, 26 and 17 days treating synthetic wastewater. The ammonium removal efficiency exceeded 94%, while the total nitrogen removal efficiency was higher than 70% at different SRTs applied. The main nitrogen removal mechanism was through nitrification/denitrification, followed by algal cell synthesis and bacterial nitrogen growth requirements. Shorter SRTs favoured the nitrification/denitrification processes over the assimilation of nitrogen by algae. The highest volumetric ammonium removal rate observed was 2.12 mgNH4+-N L−1 h−1 at an SRT of 17 d. The total gross oxygen production at the different SRTs ranged between 0.2 and 0.3 kg O2 m−3 d−1, reaching the highest production at a 52 d SRT. The differences in oxygen production between the different SRTs are attributed to the algal biomass content. The oxygen consumption decreased at shorter SRTs due to a decrease in the respiration of the microalgal-bacterial biomass. This study showed that the SRT is a key operational parameter that allows controlling the nutrient removal processes and growth of microalgal-bacterial consortia.

Acute and chronic toxicity of nickel to nitrifiers at different temperatures

This study investigated the acute nickel toxicity on nitrification of low ammonia synthetic wastewater at 10, 23, and 35°C. The nickel inhibition half-velocity constants (KI,Ni) for ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) based on Ni/MLSS ratio at 10, 23, and 35°C were 5.4 and 5.6 mg Ni/g MLSS, 4.6 and 3.5 mg Ni/g MLSS, and 9.1 and 2.7 mg Ni/g MLSS, respectively. In addition, chronic toxicity of nickel to nitrification of low ammonia synthetic wastewater was investigated at 10°C in two sequencing batch reactors (SBRs). Long-term SBRs operation and short-term batch tests were comparable with respect to the extent of inhibition and corresponding Ni/MLSS ratio. The μmax, b, and Ko of AOB were 0.16 day-1, 0.098 day-1 and 2.08 mg O2/L after long-term acclimatization to nickel of 1 mg/L at 10°C, high dissolved oxygen (DO) (7 mg/L) and long solids retention time (SRT) of 63–70 days. Acute nickel toxicity of nitrifying bacteria was completely reversible.

Does the feeding strategy enhance the aerobic granular sludge stability treating saline effluents?

The development and stability of aerobic granular sludge (AGS) was studied in two Sequencing Batch Reactors (SBRs) treating fish canning wastewater. R1 cycle comprised a fully aerobic reaction phase, while R2 cycle included a plug-flow anaerobic feeding/reaction followed by an aerobic reaction phase. The performance of the AGS reactors was compared treating the same effluents with variable salt concentrations (4.97–13.45 g NaCl/L) and organic loading rates (OLR, 1.80–6.65 kg CODs/(m3·d)). Granulation process was faster in R2 (day 34) than in R1 (day 90), however the granular biomass formed in the fully aerobic configuration was more stable to the variable feeding composition. Thus, in R1 solid retention times (SRT), up to 15.2 days, longer than in R2, up to 5.8 days, were achieved. These long SRTs values helped the retention of nitrifying organisms and provoked the increase of the nitrogen removal efficiency to 80% in R1 while it was approximately of 40% in R2. However, the presence of an anaerobic feeding/reaction phase increased the organic matter removal efficiency in R2 (80–90%) which was higher than in R1 with a fully aerobic phase (75–85%). Furthermore, in R2 glycogen-accumulating organisms (GAOs) dominated inside the granules instead of phosphorous-accumulating organisms (PAOs), suggesting that GAOs resist better the stressful conditions of a variable and high-saline influent. In terms of AGS properties an anaerobic feeding/reaction phase is not beneficial, however it enables the production of a better quality effluent.

Degradation kinetics of toilet paper fiber during wastewater treatment: Effects of solid retention time and microbial community

Toilet paper (TP) fiber is a major slowly biodegradable constituent of municipal wastewater. Oftentimes, they cannot be efficiently removed during the preliminary treatment of wastewater treatment plants (WWTPs), especially for those without installation of primary clarifiers. Although fine-mesh sieves are effective in removing fibers, their applications are not common in current WWTPs. Therefore, it is critical to understand the degradation of TP fibers during the secondary wastewater treatment. In this study, the effects of solids retention time (SRT) and microbial community on the hydrolysis kinetics of TP were investigated in sequencing-batch activated sludge systems. Meanwhile, the influence of the addition of readily biodegradable organics such as glucose on fiber degradation was also studied. It was found that the overall degradation of TP linearly increased versus SRT. At 40-day SRT, approximately 83% of TP was degraded. The hydrolysis coefficient at 40-day SRT (0.116–0.137 d−1) significantly increased compared to those at SRTs smaller than 20 days (0.025–0.034 d−1), which was probably due to the stepwise hydrolysis of cellulose and hemicellulose fibers, as well as the synergistic enzymatic activities owing to the enhanced microbial diversity at larger SRTs. Actively functioning fiber-degrading Cellvibrio genus was identified as the dominant driver of fiber hydrolysis. Interestingly, it was discovered that fiber-degrading microbial communities were inefficient at consuming either glucose or acetate. Therefore, the addition of these substrates did not enhance TP degradation.

Reducing bacterial aerosol emissions from membrane bioreactors: The impact of SRT and the addition of PAC and calcium

Bacterial aerosols resulting from membrane bioreactor (MBR) processes, which require excessive aeration in a confined space, are important to investigate because of their possible adverse effects on human health. This study investigated the influence of solid retention time (SRT) on bacterial aerosols from MBRs. Moreover, powdered activated carbon (PAC) and calcium were used to attenuate bacterial aerosol emissions from MBRs. The particulate matter (PM) emitted from the MBRs was reduced by 30.5 and 25.2% at SRTs of 20 and 80 d, respectively, compared to the level emitted at an SRT of 10 d. Total cell counts were similarly reduced at SRTs of 20 and 80 d. Longer SRTs also led to greater reductions in the particle size distribution of the sludge within 10 μm. Several factors in the MBR influenced the behavior of the bacterial aerosol emissions from the MBRs. This study showed that changes in viscosity and particle size induced by the SRT influenced the bacterial aerosol emissions in MBRs. Therefore, SRT was identified as an important design parameter affecting bacterial aerosol emissions in MBR processes. The amounts of particulate matter and bacterial aerosols were reduced in MBRs using PAC and calcium, both of which exerted an immediate effect on the bacterial aerosol emissions in MBRs by increasing the aerosol-particle size.

Linking solids retention time to the composition, structure, and hydraulic resistance of biofilms developed on support materials in dynamic membrane bioreactors

Biofilms have permeable structures, which can be used as membranes for solids separation, e.g., dynamic membrane bioreactors (DMBRs) that relied on the biofilm developed on the support material with large apertures for filtration. This study found that the biofilm formed on the support material in DMBRs under different solids retention time (SRTs) had diverse structures, which in turn determined the hydraulic resistance and filtration performance. At 5-day SRT, a thin but compact gel-like biofilm layer (porosity = 27.5%) was formed on the support materials, which had a higher hydraulic resistance of approximately 4.9 × 1011 m−1. As a result, the transmembrane pressure (TMP) rose dramatically every 20–40 days' operation. A thick but porous biofilm layer (porosity > 60% and hydraulic resistance < 2.5 × 109 m−1) was formed at longer SRTs of 20 and 40 days and the TMP could keep consistently low (<20 Pa) for more than 180 days. Therefore, a longer SRT could facilitate the formation of a porous biofilm layer on the support material, which was of critical importance for achieving long-term and low-pressure filtration. The biofilm porosity was negatively correlated with the protein/polysaccharide ratio of its extracellular polymeric substance (EPS). Extending SRT would reduce the Live/Dead cell ratio of biofilm, thus lowering EPS production while increasing the porosity. Moreover, extending SRT promoted the growth of protozoa (e.g., Euglypha) that inhabited the biofilm, which could maintain the biofilm porous and mitigate the support layer biofouling though feeding and movement.

Comparative Study of Balancing SRT by Using Modified ASM2d in Control and Operation Strategy at Full-Scale WWTP

Detailed knowledge on the composition of the influent going into the wastewater treatment system is essential for the development of a reliable computer model. In the context of WWTPs (wastewater treatment plants), the wastewater characteristics are not only important for activated sludge system modelling, but also have an impact on the appropriate control of single unit operations. The aim of this study was to evaluate the concepts of COD (chemical oxygen demand) fractionation measurement in municipal wastewater with a respirometric method in control, and modelling the biological treatment processes at WWTP using the modified Activated Sludge Model no. 2d (ASM2d) developed by Drewnowski and Makinia. The batch OUR (oxygen uptake rate) test results and COD measurements obtained at BNR plant (96,000 m3/d) in Gdansk (Poland), were compared and evaluated with the main BNR (biological nutrient removal) WWTP (144,000 m3/d) located in Malaga (Spain). Respirometric tests and COD fractionation provided the experimental database for the comparison of the wastewater characteristics and model predictions at both large WWTPs. Some parameters, such as the heterotrophic growth yield (YH) coefficient, required calibration/validation of the range (YH = 0.64 and 0.74 gCOD/gCOD for Gdańsk and Malaga WWTP, respectively) to fit the modified ASM2d. The crucial issue when dealing with the newly developed model and proposed wastewater characterization for both study plants were extremely low and high values of the XS/XI ratio, which can be used to control full-scale WWTP and balance the solid retention time (SRT) in activated sludge systems.