Effect of increasing the surface area of primary sludge on anaerobic digestion at low temperature

Two sets of reactors were operated at 15 °C and at sludge retention times (SRTs) of 65 days and 30 days, respectively. Each set was operated at six different mixing velocities. Shear forces provided by mixing affected particle size distribution and resulted in different sludge surface areas. The aim was to investigate the effect of increasing primary sludge surface area on anaerobic digestion at low temperature. The maximum surface areas at the applied mixing velocities were 5926 cm²/cm³of sludge and 4672 cm²/cm³ of sludge at SRTs of 65 days and 30 days, respectively. The corresponding calculated methanogenesis were 49% and 15% at SRTs of 65 days and 30 days, respectively. Maximum total solids (TS) reductions were 26% and 11% at 65 days and 30 days SRTs, respectively. Lipase activity increased with increasing SRT and sludge surface area. Results clearly showed that increasing sludge surface area improved sludge digestion at 15 °C.     

The effects of hydraulic retention time and sludge retention time on the fate of di-(2-ethylhexyl) phthalate in a laboratory-scale anaerobic-anoxic-aerobic activated sludge system

A laboratory-scale anaerobic-anoxic-aerobic (AAA) activated sludge wastewater treatment system was employed to investigate the effects of hydraulic retention time (HRT) and sludge retention time (SRT) on the removal and fate of di-(2-ethylhexyl) phthalate (DEHP). In the range from 5 to 14h, HRT had no significant effect on DEHP removal. However, longer HRT increased DEHP accumulation in the system and DEHP retention in the waste sludge. When SRT was increased from 15 to 25d, DEHP removal efficiency stayed above 96%. Compared to the removal of only 88% at SRT of 10d, longer SRT enhanced DEHP degradation efficiency. The optimal HRT and SRT for both nutrients (nitrogen and phosphorus) and DEHP removal were 8h and 15d. At these retention times, about 71% of DEHP was degraded by the activated sludge process, 26% was accumulated in the system, 2% was released in the effluent, and 1% remained in the waste sludge. The anaerobic, anoxic and aerobic reactors were responsible for 15%, 19% and 62% of the overall DEHP removal, respectively.     

Improved phosphate removal by selective sludge discharge in aerobic granular sludge reactors

Two lab-scale aerobic granular sludge sequencing batch reactors were operated at 20 and 30°C and compared for phosphorus (P) removal efficiency and microbial community composition. P-removal efficiency was higher at 20°C (>90%) than at 30°C (60%) when the sludge retention time (SRT) was controlled at 30 days by removing excess sludge equally throughout the sludge bed. Samples analyzed by fluorescent in situ hybridization (FISH) indicated a segregation of biomass over the sludge bed: in the upper part, Candidatus Competibacter phosphatis (glycogen-accumulating organisms--GAOs) were dominant while in the bottom, Candidatus Accumulibacter phosphatis (polyphosphate-accumulating organisms--PAOs) dominated. In order to favour PAOs over GAOs and hence improve P-removal at 30°C, the SRT was controlled by discharging biomass mainly from the top of the sludge bed (80% of the excess sludge), while bottom granules were removed in minor proportions (20% of the excess sludge). With the selective sludge removal proposed, 100% P-removal efficiency was obtained in the reactor operated at 30°C. In the meantime, the biomass in the 30°C reactor changed in color from brownish-black to white. Big white granules appeared in this system and were completely dominated by PAOs (more than 90% of the microbial population), showing relatively high ash content compared to other granules. In the reactor operated at 20°C, P-removal efficiency remained stable above 90% regardless of the sludge removal procedure for SRT control. The results obtained in this study stress the importance of sludge discharge mainly from the top as well as in minor proportions from the bottom of the sludge bed to control the SRT in order to prevent significant growth of GAOs and remove enough accumulated P from the system, particularly at high temperatures (e.g., 30°C).     

Is sludge retention time a decisive factor for aerobic granulation in SBR?

This study investigated the role of sludge retention time (SRT) in aerobic granulation under negligible hydraulic selection pressure. Results showed that no successful aerobic granulation was observed at the studied SRTs in the range of 3-40 days. A comparison analysis revealed that hydraulic selection pressure in terms of the minimum settling velocity would be much more effective than SRT for enhancing heterotrophic aerobic granulation in sequencing batch reactor (SBR). It was shown that SRT would not be a decisive factor for aerobic granulation in SBR.     

Influence of solids retention time on membrane fouling: characterization of extracellular polymeric substances and soluble microbial products

The objective of this study was to investigate the influence of solids retention time (SRT) on membrane fouling and the characteristics of biomacromolecules. Four identical laboratory-scale membrane bioreactors (MBRs) were operated with SRTs for 10, 20, 40 and 80 days. The results indicated that membrane fouling occurred faster and more readily under short SRTs. Fouling resistance was the primary source of filtration resistance. The modified fouling index (MFI) results suggested that the more ready fouling at short SRTs could be attributed to higher concentrations of soluble microbial products (SMP). Fourier transform infrared (FTIR) spectra indicated that the SRT had a weak influence on the functional groups of the total extracellular polymeric substances (TEPS) and SMP. However, the MBR under a short SRT had more low-molecular-weight (MW) compounds (<1 kDa) and fewer high-MW compounds (>100 kDa). Aromatic protein and tryptophan protein-like substances were the dominant groups in the TEPS and SMP, respectively.     

Fractionation of proteins and carbohydrates of extracellular polymeric substances in a membrane bioreactor system

The major operational problem associated with membrane bioreactors (MBR) is membrane fouling, for which extracellular polymeric substances (EPS) are primarily responsible. In this work both the soluble and bound EPS (i.e. SMP and EPS) produced in an MBR system operating under sludge retention times (SRT) of 10, 15, 20 and 33 days were fractionized by means of membranes having variable molecular weight cutoffs (300 kDa, 100 kDa, 10 kDa & 1 kDa). The results show that increasing the SRT leads to a reduction of SMP and EPS and that these reductions are more pronounced for the SRTs in the range 10–20 days. This reduction is more significant for carbohydrates than for proteins. The decrease of SMP and EPS with increasing SRT from 10 to 20 days led to a significant decrease of the level of fouling. The further increase of SRT to 33 days did not significantly impact on the level of fouling as the SMP and EPS concentrations did not change much.     

Study of Biological Activity and Process Stability in Submerged Membrane Bioreactors

Synthetic wastewater was treated in a bench scale submerged membrane bioreactor (SMBR). A long‐term experiment was conducted by varying the sludge residence time (SRT) (10–500 d) and BOD loading (1.3–0.25 kg/m³·d). The biological activity was observed in terms of the oxygen utilization rate (OUR) and adenosine triphosphate (ATP) profile; the process stability was analyzed based on the extent of organic degradation and suction pressure. The microbial population in the SMBR was dependent on the SRT and BOD loading, and its biological activity was increased with an increase in the SRT or BOD loading. At a low feed to microorganism (F/M) ratio (0.06 kg BOD/kg MLSS·d), the sludge production of the reactor was reduced to 0.04 kg MLSS/kg BOD, which is much less than in the conventional activated sludge process (0.4–0.6 kg MLSS/kg BOD). The F/M ratio influenced the biological activity (via ATP and the OUR) significantly at a short SRT (≤90 d). However, the effect of the F/M ratio ceased at a low F/M ratio (≤ 0.07 kg BOD/kg MLSS·d). The accumulation of organics in the SMBR was accompanied with an increase in the supernatant TOC, which caused a high suction pressure and an abrupt change in the operating conditions to process instability. However, the process stability of the SMBR increased with an increase in the SRT and a decrease in the BOD loading along with a concomitant decrease in the biological activity and sludge production.     

Nitrogen transformations during aerobic/anoxic sludge digestion

Laboratory experiments were conducted to study and compare nitrogen transformations occurring under both aerobic digestion and aerobic/anoxic (A/A) digestion. The process performance was examined at different sludge residence times (SRTs), temperatures and anoxic cycles. Both modes of operation gave comparable solids reduction results. However, introduction of anoxic periods to aerobic sludge digestion appears to be a promising alternative to control pH during digestion through endogenous nitrate respiration (ENR). Operating an aerobic digester with an anoxic phase to achieve complete denitrification would also improve supernatant quality over that achieved solely by aerobic digestion. Alternating A/A operation can conserve most of the influent alkalinity and maintain near neutral pH condition over prolonged periods. The A/A digestion of mixed primary/waste-activated sludge achieved up to 43.7% reductions in volatile suspended solids, 33.7% removal of total nitrogen, and a specific ENR rate of 5.75 x 10(-2) mg NO3-N/mg VSSd. Optimum results were obtained at 10 d SRT, 30 degrees C temperature, and 50% anoxic cycle length.     

Low temperature thermo-chemical pretreatment of dairy waste activated sludge for anaerobic digestion process

An investigation into the influence of low temperature thermo-chemical pretreatment on sludge reduction in a semi-continuous anaerobic reactor was performed. Firstly, effect of sludge pretreatment was evaluated by COD solubilization, suspended solids reduction and biogas production. At optimized condition (60 °C with pH 12), COD solubilization, suspended solids, reduction and biogas production was 23%, 22% and 51% higher than the control, respectively. Secondly, semi-continuous process performance was studied in a lab-scale semi-continuous anaerobic reactor (5 L), with 4 L working volume. With three operated SRTs, the SRT of 15 days was found to be most appropriate for economic operation of the reactor. Combining pretreatment with anaerobic digestion led to 80.5%, 117% and 90.4% of TS, SS and VS reduction respectively, with an improvement of 103% in biogas production. Thus, low temperature thermo-chemical can play an important role in reducing sludge production.     

Disturbance and temporal partitioning of the activated sludge metacommunity

The resilience of microbial communities to press disturbances and whether ecosystem function is governed by microbial composition or by the environment have not been empirically tested. To address these issues, a whole-ecosystem manipulation was performed in a full-scale activated sludge wastewater treatment plant. The parameter solids retention time (SRT) was used to manipulate microbial composition, which started at 30 days, then decreased to 12 and 3 days, before operation was restored to starting conditions (30-day SRT). Activated sludge samples were collected throughout the 313-day time series in parallel with bioreactor performance (‘ecosystem function''). Bacterial small subunit (SSU) rRNA genes were surveyed from sludge samples resulting in a sequence library of >417 000 SSU rRNA genes. A shift in community composition was observed for 12- and 3-day SRTs. The composition was altered such that r-strategists were enriched in the system during the 3-day SRT, whereas K-strategists were only present at SRTs⩾12 days. This shift corresponded to loss of ecosystem functions (nitrification, denitrification and biological phosphorus removal) for SRTs⩽12 days. Upon return to a 30-day SRT, complete recovery of the bioreactor performance was observed after 54 days despite an incomplete recovery of bacterial diversity. In addition, a different, yet phylogenetically related, community with fewer of its original rare members displaced the pre-disturbance community. Our results support the hypothesis that microbial ecosystems harbor functionally redundant phylotypes with regard to general ecosystem functions (carbon oxidation, nitrification, denitrification and phosphorus accumulation). However, the impacts of decreased rare phylotype membership on ecosystem stability and micropollutant removal remain unknown.     

Mass balance of nitrogen, and estimates of COD, nitrogen and phosphorus used in microbial synthesis as a function of sludge retention time in a sequencing batch reactor system

This study investigated characteristics of a sequencing batch reactor (SBR) system which was varied with respect to sludge retention time (SRT) (5.9, 8.2, 10.5, 12.2, and 16.2 days). The removal efficiencies of chemical oxygen demand (COD) were more than 90% under all SRT conditions, and the greatest efficiency (92.2%) occurred with a SRT of 16.2 days. As the SRT increased, the denitrification rate per mixed liquor suspended solids (MLSS) during the anoxic(I) period decreased significantly from 166.3 mg NO(X)(-)-N/g MLSS d to 68.8 mg NO(X)(-)-N/g MLSS d. As the SRT increased, the phosphorus removal efficiency decreased from 47.1% (SRT of 5.9 days) to 31.0% for a SRT of 16.2 days, because active phosphate release and uptake occurred under shorter SRT conditions. The mass balance of nitrogen (with respect to nitrogen in the influent) at a SRT of 16.2 days (the highest nitrogen removal efficiency) showed 14.9% of nitrogen was removed in clarified water effluent, 49.7% was removed by the sludge waste process and 33.3% was removed by denitrification. Nitrogen processing was well accounted for in the SBR system as the nitrogen mass balance was close to 100% (97.9%).     

Comparison of polyhydroxyalkanoates production by activated sludges from anaerobic and oxic zones of an enhanced biological phosphorus removal system: effect of sludge retention time

This study compared the PHAs production behavior of sludges from the anaerobic and oxic phases of an enhanced biological phosphorus removal (EBPR) system. This was accomplished by using the kinetics and stoichiometric coefficients obtained from aerobic batch tests to evaluate the performance of these two sludges. Experimental results indicated that the metabolic behavior of the sludges for PHAs production depend significantly on the operating sludge retention time (SRT) of the EBPR system. The oxic sludge with 5 days of SRT exhibited better PHAs production performance than anaerobic sludge. Conversely, the anaerobic sludge with 15 days of SRT had superior PHAs production capability compared to oxic sludge. These comparisons suggest that whether anaerobic or oxic sludge should be employed for PHAs production depends mainly on the operating SRT of the EBPR system.     

Effect of low solids retention time and focused pulsed pre-treatment on anaerobic digestion of waste activated sludge

The interacting effects of Focused Pulsed (FP) treatment and solids retention time (SRT) were evaluated in laboratory-scale digesters operated at SRTs of 2-20 days. Anaerobic digestion and methanogenesis of waste activated sludge (WAS) were stable for SRT ? 5 days, but the effluent soluble organic compounds increased significantly for SRT = 2 days due to a combination of faster hydrolysis kinetics and washout of methanogens. FP treatment increased the CH₄ production rate and TCOD removal efficiency by up to 33% and 18%, respectively, at a SRT of 20 days. These effects were the result of an increase in the hydrolysis rate, since the concentrations of soluble components remained low for SRT ? 5 days. Alternately, FP pre-treatment of WAS allowed the same conversion of TCOD to CH₄ with a smaller SRT and digester size: e.g., 40% size savings with a CH₄ conversion of 0.23 g CH₄-COD/g COD_in.     

Removal of Cu(II) ions by biosorption onto powdered waste sludge (PWS) prior to biological treatment in an activated sludge unit: A statistical design approach

Biological treatment of synthetic wastewater containing Cu(II) ions was realized in an activated sludge unit with pre-adsorption of Cu(II) onto powdered waste sludge (PWS). Box-Behnken experimental design method was used to investigate Cu(II), chemical oxygen demand (COD) and toxicity removal performance of the activated sludge unit under different operating conditions. The independent variables were the solids retention time (SRT, 5–30 d), hydraulic residence time (HRT, 5–25 h), feed Cu(II) concentration (0–50 mg L^?1) and PWS loading rate (0–4 g h^?1) while percent Cu(II), COD, toxicity (TOX) removals and the sludge volume index (SVI) were the objective functions. The data were correlated with a quadratic response function (R² = 0.99). Cu(II), COD and toxicity removals increased with increasing PWS loading rate and SRT while decreasing with the increasing feed Cu(II) concentration and HRT. Optimum conditions resulting in maximum Cu(II), COD, toxicity removals and SVI values were found to be SRT of 30 d, HRT 15 h, PWS loading rate 3 g h^?1 and feed Cu(II) concentration of less than 30 mg L^?1.     

Leaching of iron and toxic heavy metals from anaerobically-digested sewage sludge

Summary Heavy metal-loaded sewage sludge was leached abiotically using FeCl₂ and FeCl₃ which are applied in waste water treatment plants to eliminate phosphate and for coagulation. Due to the hydrolyzing nature of ferric iron, ferric chloride (100 mmll L^–1) was able to solubilize more than 90% of copper and zinc and more than 80% of cadmium, with an optimal pulp density of 3% (w/v), after 10 h of exposition at 25°C. Chromium, lead and nickel were solubilized to an extent of 40–70%. With the exception of copper (redoxolysis), all heavy metals monitored were leached following the principle of acidolysis. Chemical leaching with iron resulted in a secondary contamination of sewage sludge (96 g iron per kg dry weight). The insoluble iron compounds which were precipitated for adsorbed to sludge flocks could be resolubilized with oxalic acid (100 mM, pH<3.3) up to an extent of 90%. Iron was leached by acidolysis and held in solution by complexation with oxalic acid. The pH optimum for the treatment of sewage sludge with 100 mmol L^–1 oxalic acid was pH 3.3. At this pH an excessive solubilization of nutrient elements and compounds (phosphorus, nitrogen, alkali and alkali earth elements) could be avoided concomitantly leaching 75% iron. Furthermore the hydrophobicity of the sewage sludge was significantly reduced as a result of treatment with iron chloride.Thiobacillus ferrooxidans (isolated from arsenopyrite and adapted on sewage sludge) utilized ferrous iron as an energy source in the presence of chloride ions (FeCl₂) as efficiently as ferrous sulphate. No toxic effects of oxalic acid onT. ferrooxidans were observed at the prevailing concentration.     

Microbial behaviors involved in cake fouling in membrane bioreactors under different solids retention times

Biomass characteristics and membrane performances in the MBRs operated at a high flux of 30 L/m² h under different SRTs (10, 30 days, and infinity) were monitored. Results showed that more serious cake-fouling happened in the SRT-infinity MBR, which correlated with the activated sludge characteristics such as smaller floc size and greater EPS amount. DGGE analysis indicated that the microbial community shifted in different ways under various SRTs, which also influenced EPS productions in the MBRs. Different microbial communities were developed on the membrane surfaces at various operating stages and SRTs. Possibly, the activated sludge characteristics (such as MLSS concentration, EPS properties) and hydrodynamic conditions influenced by the SRTs were associated with cake layer development and membrane fouling propensity. Insight into the EPS characteristics and deposition behaviors of bacterial flocs will be crucial to explore appropriate biofouling control strategies in MBRs.     

Modeling of simultaneous growth and storage kinetics variation under unsteady feast conditions for aerobic heterotrophic biomass

The heterotrophic biomass has the capacity of utilizing substrate predominantly for growth or storage processes under steady-state conditions. In this study, the short-term variations in growth and storage kinetics of activated sludge under disturbed feeding conditions were analyzed using a multi-component biodegradation model. The variations in growth and storage kinetics were investigated with the aid of multi-response modeling and identifiability analysis. It was found that the heterotrophic biomass is able to increase its direct growth activity together with reducing the substrate storage capability under the availability of external substrate. Reducing the sludge age (SRT) from 10 to 2?days increased the maximum specific growth rate, μ (OHO,Max) from 3.9 to 7.0 day(-1), but did not considerably affected the maximum storage rate, k (Stor,OHO). The alteration of sludge age also elevated the half-saturation constant for growth (K (S,OHO)) from 5 to 25?mg COD/L. The increase in primary growth metabolism together with reduced storage rate was validated by model for two different sludge ages in the availability of external substrate. Aside from having a lower storage capability, the biomass had fast adaptation ability to direct growth process at low SRTs. The alteration of feed conditions was found to have different impacts on storage and growth kinetics. These results are significant and advance the field of activated sludge modeling under dynamic conditions by incorporation of short-term effects. Appropriate modifications including short-term effects in model structure may also reduce dynamic model recalibration efforts in the future.     

Chemical and physical effects on the shape and growth of the diatomBiddulphia sinensis Greville in batch cultures: A contribution to bioindication in plankton ecology

Summary Different influences, like changes in salinity and temperature, the effects of heavy metals like copper and mercury, and the influences of nitrate and silicate deficiencies on the shape, generation time, and yield of the marine plankton diatomBiddulphia sinensis were tested in batch cultures. Those cells which show multiple or missing setae are classified as seta-aberrant, teratological forms. Sudden changes in salinity and temperature (30–15%. S; 12–20°C) lead to no more than 5% of aberrant cells (control cultures below 1%). Heavy metals (copper and mercury) cause a significant increase of the generation time, a distinct decrease of the cell yield, and (in the case of copper) up to 50% of seta-aberrant cells. The cells die at concentrations of copper above 10^–5M.l^–1, though EDTA was added as chelant. At concentrations below 10^–5M.l^–1 of copper, addition of EDTA as well as an increase of temperature from 12 to 20°C cause a reduction of toxicity. During batch growth the cells adapt to mercury but not to copper. At mercury concentrations above 10^–8M.l^–1 the cells die, even if EDTA is added. Unlike in copper-treated medium, EDTA does not reduce the toxicity of mercury. However, a detoxicating effect (20 instead of 12°C) can be observed due to higher temperature. Silicate deficiency, like the influence of copper and mercury leads to an elongation of the pervalvar axes. Particularly at a temperature of 20°C bended pervalvar axes and sometimes lateral evaginations of the frustule will be formed; this does not happen under the influence of copper and mercury. Multiple setae never appear under silicate deficiency. Under nitrate deficiency more than 10% of the cells are seta-aberrant. The suitability ofBiddulphia sinensis for test experiments in shipboard cultures is shown. Water samples were tested on the lack of nitrate, silicate, and phosphate. The percentage of seta-aberrant cells, the generation times, and the yields were determined. Simultaneous lack of phosphate and nitrate causes up to 30% of teratological cells. The seta-aberrant cells always appear during the exponential growth phase.     

Experiences with the dual digestion of municipal sewage sludge

The dual digestion process was investigated using sludge samples collected at the WWTP of Tomaszow Mazowiecki (Poland). Mixed sludge was treated in a laboratory setup under batch and semi-continuous conditions. Dual digestion with a 1d SRT aerobic thermophilic pretreatment followed by an anaerobic step with 20 d of SRT turned out to be optimal, since a 44-46% VS reduction and a biogas yield of 480 dm(3)/kg VS fed were achieved. In the course of the process, the concentration of nitrogen in supernatant increased over 5 times and its major portion was converted into ammonia. Phosphorus also entered the supernatant, reaching over 200 g/m(3). The dual digestion noticeably deteriorated the sludge dewaterability. Following completion of the process, capillary suction time measurements averaged 64 s for the raw sludge, 400 s for aerobically pretreated sludge and 310-360 for the anaerobically digested sludge. Aerobic pretreatment consistently reduced Enterobacteriaceae content to below detectable limits.     

聚磷菌和聚糖菌的竞争影响因素研究进展

目前, 强化生物除磷工艺(EBPR)以其经济有效而得到广泛的应用, 该工艺关键在于聚磷菌的富集。然而已经发现, 有一类细菌—聚糖菌(GAOs)能够和聚磷菌(PAOs)竞争, 导致除磷效果恶化。关于PAOs-GAOs的竞争, 研究已经很多, 但是其结论有趋同也有矛盾, 有必要对此进行分析讨论。根据近年来国内外的相关报道, 阐述了聚磷菌与聚糖菌的竞争影响因素, 其中碳磷比、碳源种类、温度、pH值是关键因素, 而污泥龄、溶解氧以及水力停留时间等因素对于PAOs和GAOs的竞争也起一定的作用。此外, 在EBPR系统中, 缺氧条件下, 存在反硝化聚磷菌(DPB)和反硝化聚糖菌(DGAO)也会对聚磷菌富集和系统除磷产生影响。最后对EBPR系统未来的发展方向进行了展望。