Effects of temperature transient conditions on aerobic biological treatment of wastewater

The effects of temperature variations on aerobic biological wastewater treatment were evaluated with respect to treatment efficiency, solids discharges, sludge physicochemical properties and microbiology. The effects of controlled temperature shifts (from 35 degrees to 45 degrees C; from 45 degrees to 35 degrees C) and periodic temperature oscillations (from 31.5 degrees C to 40 degrees C, 6-day period, for 30 days) were assessed in 4 parallel, lab-scale sequencing batch reactors (SBRs) that treated pulp and paper mill effluent. Overall, the temperature shifts caused higher effluent suspended solids (ESS) levels (25-100 mg/L) and a decrease (up to 20%) in the removal efficiencies of soluble chemical oxygen demand (SCOD). Lower ESS levels were triggered by a slow (2 degrees C/day) versus a fast (10 degrees C/12h) temperature shift from 35 degrees to 45 degrees C, but the SCOD removal efficiencies decreased similarly in both cases (from 66+/-3% and 65+/-2% to 49+/-3% and 51+/-3%). Temperature oscillations caused an increased deterioration of the sludge settleability [high sludge volume indices (SVI); low zone settling velocities (ZSV)], high ESS levels and lower SCOD removals. The temperature transients were associated with poor sludge settleability (SVI>100 mL/g MLSS, ZSV<1 cm/min), more negatively charged sludge (up to -0.35+/-0.03 meq/g MLSS), increased filament abundance (approximately 4 to 4.5, subjective scale equivalent to very common), and decreased concentrations of protozoa and metazoa (25,000-50,000 microorganisms/mL sludge). The controlled, periodic temperature oscillations had a slight impact on SCOD removal efficiency (5% decrease), and did not seem to select for robust microorganisms that withstood the temperature shift. Sludge deflocculation and filament proliferation caused by these temperature transients may explain the higher ESS levels.     

Microbial community structure in activated sludge floc analysed by fluorescence in situ hybridization and its relation to floc stability

The efficiency of activated sludge treatment plants is dependent on the solid-liquid separation properties of the activated sludge. A critical parameter is the stability of the microbial flocs. Weak flocs deflocculate easily leaving increased concentrations of suspended solids in the effluent. The knowledge about how different bacteria are attached to the flocs and their influence on the bioflocculation is limited. In this study, the deflocculation of different phylogenetic groups of bacteria in activated sludge from a full scale plant was investigated. The experiments were carried out by using a shear method where the sludge flocs are deflocculated under controlled shear conditions. The degree of deflocculation was measured as increase in turbidity of the supernatant. Identification and quantification of the microbial community structure of both total activated sludge and deflocculated bacteria were conducted with group-specific gene probes for broad groups of bacteria (Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria and Actinobacteria) and fluorescence in situ hybridization (FISH). The microbial community structure of the deflocculated bacteria was different compared to the total activated sludge with a higher abundance of Gammaproteobacteria in the supernatant indicating that different groups of bacteria are bound with different strength to the floc surface. The results show that the bacteria in the outer parts of the flocs are relatively loosely bound to the floc matrix and can be easily eroded from the surface when exposed to shear.     

Anaerobic treatment of phenol in wastewater under thermophilic condition

Over 99% of phenol was effectively degraded in an upflow anaerobic sludge blanket (UASB) reactor at 55 degrees C with 40 h of hydraulic retention time (HRT) for a wastewater containing 630 mg/L of phenol, corresponding to 1500 mg/L of chemical oxygen demand (COD) and a loading rate of 0.9 g-COD/L/d. The maximum specific methanogenic activity (SMA) of the phenol-degrading sludge was 0.09 g-CH4-COD/g-volatile suspended solids (VSS)/d. Based on 16S rDNA analysis, a total of 21 operational taxonomy units (OTUs) were found in the sludge, of which eight (42.6% of the total population) were related to the sequences in the GenBank with similarity of over 97%, and 13 (79.6%) were affiliated with the known thermophilic species. Additional SMA data and phylogenetic analysis suggest that the degradation pathway of phenol for thermophilic sludge was likely via caproate, instead of benzoate as for the mesophilic sludge.     

The Application of Cross-flow Microfiltration Technology to the Concentration of Sewage Works Sludge Streams

A LOCALLY DEVELOPED cross-flow microfiltration process using woven fabric tubes was used to concentrate waste activated sludge and anaerobic digested sludge from a conventional sewage works.
Results showed that waste activated sludge could be concentrated from 5 to 50 g/l total solids. The permeate quality was good (0 to 50 mg/l suspended solids), but deteriorated both with time and increasing feed solids concentration.
A cross-flow microfilter was coupled to a pilot scale anaerobic digester and the digester solids were increased from 26 to 55 g/l total solids. The organic loading to the digester was increased from 1.8 to 3.1 kg volatile solids per m³ per day. The solids retention time was held constant at 26 days, while the liquid retention time was decreased from 26 to 14 days. The permeate quality was significantly better than the supernatant liquor from a comparable digester (suspended solids 122 and 570 mg/l, respectively).
Data obtained from the cross-flow microfiltration of waste activated sludge was used to regress for the constants in a mathematical model of steady-state flux.     

Anaerobic degradation of dimethyl phthalate in wastewater in a UASB reactor

Over 99% of dimethyl phthalate (DMP) and 93% of chemical oxygen demand (COD) were effectively removed in a continuous upflow anaerobic sludge blanket (UASB) reactor from a wastewater containing 600mg/L DMP at 8h of hydraulic retention time (HRT), corresponding to a loading rate of 3g-COD/(Ld). Each gram of sludge, expressed as volatile suspended solids (VSS), had a maximum specific methanogenic activity (SMA) of 24mg-CH(4)/(g-VSSd) using DMP as the sole carbon source. The sludge yield was estimated as 0.08g-VSS/g-COD. During anaerobic degradation, DMP was de-esterified, first to mono-methyl phthalate (MMP) and then to phthalate, before being de-aromatized and subsequently converted to CH(4) and CO(2). The maximum specific degradation rates of DMP, MMP and phthalate were 415, 88 and 36mg/(g-VSSd), respectively. Analysis based on polymerase chain reaction (PCR) and denaturing gradient gel electrophoresis (DGGE) showed a gradual shift of microbial population with the increase of DMP loading.     

A predictive model for the reactor inorganic suspended solids concentration in activated sludge systems

A simple predictive model for the activated sludge reactor inorganic suspended solids (ISS) concentration (excluding that from chemical precipitant dosing) is presented. It is based on the accumulation of influent ISS in the reactor and an ordinary heterotrophic organism (OHO) ISS content (fiOHO) of 0.15 mg ISS/mg OHO organic (volatile) suspended solids (VSS) and a variable phosphate accumulating organism (PAO) ISS content (fiPAO) proportional to their P content (fXBGP). Organism ISS content is conceptualized as the uptake of dissolved inorganic solids by active organisms, which when dried in the total suspended solids (TSS) test procedure, precipitate and manifest as ISS. The model is validated with data from 22 investigations conducted over the past 15 years on 30 aerobic and anoxic-aerobic nitrification-denitrification (ND) systems and 18 anaerobic-anoxic-aerobic ND biological excess P removal (BEPR) systems variously fed artificial and real wastewater, and operated from 3 to 20 days sludge age. The predicted reactor VSS/TSS ratio reflects the observed relative sensitivity to sludge age, which is low, and to BEPR, which is high. To use the model for design, two parameters need to be known: (1) the influent ISS concentration, which is not commonly measured in wastewater characterization analyses and (2) the P content of PAOs (fXBGP), which can vary considerably depending on the extent of anoxic P uptake BEPR that takes place in the system. Some guidance on the measurement of influent ISS concentration and selection of the PAO P content to calculate the mixed liquor VSS/TSS ratio for design is given.     

Operational strategies for an activated sludge process in conjunction with ozone oxidation for zero excess sludge production during winter season

A pilot-scale activated sludge system coupled with sludge ozonation process was operated for 112 days of a winter season without excess sludge wasting. The concept of this process is that the excess sludge produced is first disintegrated by ozone oxidation and then recirculated to a bioreactor in order to mineralize the particulate and soluble organic compounds. The basis of operation was to determine either the optimal amount of sludge in kg SS ozonated each day (SO) or the optimal ozonation frequency under the variable influent chemical oxygen demand (COD) loading and temperature conditions, since the ozone supply consumes costly energy. The optimal SO was obtained using the theoretically estimated sludge production rate (SP) and experimentally obtained ozonation frequency (n). While the SP was mainly subject to the COD loadings, sludge concentration was affected by the temperature changes in winter season. The optimal n was observed between 2.5 and 2.7 at around 15 degrees C, but it was doubled at 10 degrees C. Mixed liquor suspended solids (MLSS) concentration was leveled off at around 5000 mg/L in bioreactor at 15 degrees C, but the volatile fraction of MLSS was fixed around 0.7 indicating that there was no significant inorganic accumulation. Suspended solids (SS) and soluble COD in effluents kept always a satisfactory level of 10 and 15 mg/L with sufficient biodegradation. It was recommended to apply a dynamic SO under variable influent COD loadings and temperature conditions to the activated sludge system without excess sludge production for saving energy as well as system stabilization.     

Elutriated acid fermentation of municipal primary sludge

The performance of a novel fermentation process, adopting a sludge blanket type configuration, for higher hydrolysis/acidogenesis of the municipal primary sludge was investigated under batch and semi-continuous conditions with varying pH and temperature. This acid elutriation slurry reactor provided higher system performance with a short HRT (5d) and higher acidogenic effluent quality under pH 9 and thermophilic (55 degrees C) conditions. The hydrolysis of the sludge was revealed to be significantly dependent on seasonal effects for sludge characteristics but with little impact on acidogenesis. Based on the rainy season at the optimal conditions, VFA production and recovery fraction (VFA(COD)/COD) were 0.18 g VFA(COD)/g VSS(COD) and 63%. As byproducts, nitrogen and phosphorus release were measured at 0.006 g N/g VSS(COD) and 0.003 g P/g VSS(COD), respectively. For the mass balance in a full-scale plant (Q=158,880 m(3)/d) based on the rainy season, the VFA and non-VFA (as COD) production were 3110 kg VFA(COD)/d and 1800 kg COD/d, resulting in an increase of organics of 31 mg COD/L and 20mg VFA(COD)/L and nutrients of 0.7 mg N/L and 0.3 mg P/L in the influent sewage. The economical benefit from this process application was estimated to be about 67 dollars per 1000 m(3) of sewage except for energy requirements and also, better benefits can be expected during the dry season. Moreover, the results revealed that the process has various additional advantages such as pathogen-free stabilized solids production, excellent solids control and economical benefits.     

Anaerobic sewage treatment in a one-stage UASB reactor and a combined UASB-Digester system

The treatment of sewage at 15 degrees C was investigated in a one-stage upflow anaerobic sludge blanket (UASB) reactor and a UASB-Digester system. The latter consists of a UASB reactor complemented with a digester for mutual sewage treatment and sludge stabilisation. The UASB reactor was operated at a hydraulic retention time of 6h and a controlled temperature of 15 degrees C, the average sewage temperature during wintertime of some Middle East countries. The digester was operated at 35 degrees C. The UASB-Digester system provided significantly (significance level 5%) higher COD removal efficiencies than the one-stage UASB reactor. The achieved removal efficiencies in the UASB-Digester system and the one-stage UASB reactor for total, suspended, colloidal and dissolved COD were 66%, 87%, 44% and 30%, and 44%, 73%, 3% and 5% for both systems, respectively. The stability values of the wasted sludge from the one-stage UASB reactor and the UASB-Digester system were, respectively, 0.47 and 0.36g CH(4)-COD/g COD. Therefore, the anaerobic sewage treatment at low temperature in a UASB-Digester system is promising.     

反应沉淀一体式矩形环流装置处理低C/N值污水

反应沉淀一体式矩形环流生物反应器(RPIR)具有污泥自动回流功能,有利于硝化菌及反硝化菌的截留和富集,且能够充分利用碳源。在常温下采用该工艺处理低C/N值城市污水,当进水COD为212~538 mg/L、C/N为2.6~10.8(平均为5.4)并控制反应区混合液的DO为1.0 mg/L左右时,就投加悬浮填料和不投加悬浮填料两种情况下的除污效果进行了对比。在HRT为4 h、MLSS为10 000 mg/L左右的条件下,当不投加悬浮填料时,反应器对COD、NH3-N和TN的平均去除率分别为84.8%、67.6%和42.3%,而投加悬浮填料以后,对这三者的平均去除率分别提高到89.9%、79.3%和63.8%,脱氮容量也从0.09提高到0.13,说明投加悬浮填料能够强化脱氮效率。     

Suspended solids abatement by pH increase—upgrading of an oxidation pond effluent

The effectiveness of solids abatement by pH increase was investigated using the jar test procedure with a bentonite tap water suspension and an urban wastewater and an oxidation pond effluent. The results indicated that, depending on the suspended particles and on the dissolved ions, pH values between 9.5 and 12 induced extensive solids elimination without adding any other chemical than a base, i.e. sodium hydroxide or lime. The major effective reactions are then the calcium carbonate precipitation and the magnesium hydroxide precipitation. Moreover, this process does not require a flocculation step but only a precipitation step where the particles are entrapped by sweep coagulation and adsorption—coagulation. A continuous reactor was operated with an oxidation pond effluent. A suspended solids concentration less than 30 mg/l was obtained by adjusting pH between 11 and 11.5 while the reactor was operated up to 20 m/h superficial upflow velocity corresponding to a residence time through the whole unit of 5 min only. The sludge settling velocity depends on pH and on the primary particles but a maximum settling velocity larger than 1 m/h is easily reached. The concentration factor is then about 100.     

Sludge accumulation pattern in an anaerobic pond under Mediterranean climatic conditions

The main objective of this study was to observe the sludge accumulation pattern of an experimental, covered, anaerobic pond treating municipal wastewater under Mediterranean climatic conditions throughout a 2-year operational period (1999-2000) in order to form a seasonal sludge accumulation model which may be used to predict the required desludging time, not only of the particular anaerobic pond used in the study, but also for other types of anaerobic ponds and operational situations. The 4-m deep pond was supplied with pre-screened, untreated wastewater from the nearby treatment plant of Thessaloniki, Greece, initially at a flow rate of 120m3/day and later at a flow rate of 150m3/day. The influent characteristics were BOD5 441 mg O2/L, COD 942 mg O2/L and suspended solids (SS) 574 mg/L. BOD5, COD, and SS concentrations of the pond effluent were reduced by 50%, 53%, and 64%, respectively, in comparison with those of the influent. During the operational period, three distinctly different zones were seen to form within the anaerobic pond: The first zone, which formed at the bottom of the pond, consisted of inert, high-density sludge. The second zone, which formed above this, contained a high concentration of volatile (easily biodegradable) sludge. The third upper zone (supernatant), was a liquid layer low in suspended solids. The accumulation of sludge in the pond followed an annual sinusoidal pattern with high values during winter and low ones during summer due to the increased digestion rate. The maximum high-density sludge height observed was 0.7m, or 2% (14 m3) of the total pond volume. The maximum volatile sludge accumulation reached 3.1 m, or 53% (300 m3) of the pond volume. A seasonal sludge accumulation model, based on the sludge inflow and seasonal digestion rates, was used to simulate the annual fluctuation in accumulation rate for the local (Mediterranean type) climatic conditions. Monthly values of accumulation (or digestion) rate of sludge (K(AS)) were experimentally estimated at specific mean monthly air temperatures and approximated by a regression second degree polynomial equation to be used with the model. The predicted desludging interval for our experimental pond was 3 years.     

The immunochemical detection of stress proteins in activated sludge exposed to toxic chemicals

The heat shock protein, GroEL, was found to be induced in activated sludge cultures exposed to perturbations of chemicals (cadmium, pentachlorophenol, and acetone) or heat stress. In laboratory activated sludge reactors, GroEL was rapidly induced (within minutes) in the presence of 5 mg/l or greater total cadmium. At 5 mg/l cadmium, however, moderate to insignificant changes in activated sludge process performance indicators [effluent suspended solids concentration, chemical oxygen demand (COD) removal, and specific oxygen uptake rate] were observed. As total cadmium concentrations increased above 5 mg/l, there was a significant and consistent increase in effluent volatile suspended solids concentrations from activated sludge sequencing batch reactors relative to unstressed controls. These results indicate that stress proteins may serve as sensitive and rapid indicators of mixed liquor toxicity which can adversely impact treatment process performance, but that GroEL may not be a good candidate protein for this purpose.     

浙江某工业废水处理厂升级改造运行效果解析

浙江某工业废水处理厂升级改造,采用AAO—MBBR复合生物膜工艺,在未新增建设用地和扩建池容的基础上,日处理量由3×10⁴m³/d提高至6×10⁴m³/d。改造后实际运行出水COD、TP、NH₃-N和TN浓度分别为(37.7±6.61)、(0.09±0.03)、(0.25±0.14)和(5.87±1.54)mg/L,出水水质稳定达到一级A标准。实际监测表明,在好氧MBBR区存在TN去除现象,约占TN总去除量的10.36%。系统内的优势硝化菌属为硝化螺旋菌属Nitrospira,其在悬浮载体生物膜和活性污泥中的相对丰度分别为8.98%和0.92%,悬浮载体的投加使硝化细菌得到有效富集;反硝化菌在生物膜中的占比为7.94%,为悬浮载体同步硝化反硝化(SND)效果的发生提供了微观保证,提高了TN去除率。     

Aerobic stabilization of primary and mixed primary-chemical (alum) sludge

This study was conducted to investigate the feasibility of using aerobic digestion as a method for the stabilization of mixed primary-chemical (alum) sludge from a physical-chemical treatment plant. Aerobic digestion was carried out in “continuous” flow (batch fed once a day) laboratory reactors with detention times ranging from 5 to 35 days held at 7, 12, 18 and 25°C.Temperature was found to have a slightly greater influence on the reduction of volatile suspended solids in primary sludge than in mixed primary-chemical sludge. Nitrification took place in the reactors treating both primary and mixed primary-chemical sludge. The high content of aluminum in the mixed primary-chemical sludge did not inhibit the nitrifiers. The oxygen-uptake rate varied between approximately 5 mg O₂ g⁻¹ VSS h⁻¹ and 1 mg O₂ g⁻¹ VSS h⁻¹ depending on the detention time and the reactor temperature. Adenosine triphosphate content per unit volatile suspended solids indicated a low content of active biomass during aerobic digestion.     

悬浮填料型活性污泥生物脱氮工艺研究

对悬浮填料型活性污泥生物脱氮工艺进行了研究。试验期间进水有机物浓度波动较大,出水COD浓度基本在30mg/L左右,COD的总平均去除率达到86.98%,氨氮平均去除率达到72.25%,出水平均为8.15mg/L;总氮的平均去除率达到48.39%,出水浓度平均为17.63mg/L。因此利用该反应器对生活污水进行脱氮处理是可行的。     

Anaerobic stabilisation and conversion of biopolymers in primary sludge--effect of temperature and sludge retention time

The effect of sludge retention time (SRT) and process temperature on the hydrolysis, acidification and methanogenesis of primary sludge was investigated in completely stirred tank reactors (CSTRs). The CSTRs were operated to maintain SRTs of 10, 15, 20 and 30 days at process temperatures of 25 degrees C and 35 degrees C. The rates of hydrolysis and the biodegradability of primary sludge were assessed in batch reactors incubated at 15 degrees C, 25 degrees C and 35 degrees C. The results revealed that the major amount of sludge stabilisation occurred between 0 and 10 days at 35 degrees C and 10 and 15 days at 25 degrees C. Hydrolysis was found to be the rate limiting-step of the overall digestion process, for the reactors operated at 35 degrees C and 25 degrees C, except for the reactor operated at 10 days and 25 degrees C. At the latter conditions, methanogenesis was the rate-limiting step of the overall digestion process. Proteins hydrolysis was limited to a maximum value of 39% at 30 days and 35 degrees C due to proteins availability in the form of biomass. The biodegradability of primary sludge was around 60%, and showed no temperature dependency. The hydrolysis of the main biopolymers and overall particulate COD of the primary sludge digested in CSTRs were well described by first-order kinetics, in case hydrolysis was the rate-limiting step. Similarly, the hydrolysis of the overall particulate COD of the primary sludge digested in batch reactors were described by first-order kinetics and revealed strong temperature dependency, which follows Arrhenius equation.     

Nitrification of high strength ammonia wastewaters: comparative study of immobilisation media

Due to legislative pressures, sludge production and processing in the UK will increase substantially in the future resulting in a supernatant liquid high in ammonia (500-1000 mg l-1) and "hard" COD (approximately 500 mg l-1). A small footprint reactor is required to effectively nitrify this effluent, and the aim of this work was to compare a number of immobilisation media under a variety of conditions in order to determine which media held the most promise for future development. Laboratory-scale continuously stirred tank reactors containing freely suspended and immobilised biomass were operated with a high-strength synthetic ammonia wastewater (500 mg N l-1) to determine the nitrification rates at various temperatures, and ammonia and COD loadings. COD:NH3 ratios in sludge liquors vary widely depending on the treatment processes employed, and therefore ratios of 1:1 and 2:1 were tested as being fairly typical. The freely suspended nitrifiers were washed out of the reactors at a 1 d hydraulic retention time (HRT), whereas the reactors containing adsorption particles (Linpor and Kaldnes) and PVA-encapsulated nitrifiers continued partially nitrifying down to 12 h, and oxygen addition enhanced nitrification. A decrease in temperature from 25 to 16 degrees C only caused a small (10%) decrease in nitrification in the immobilised cell reactors, demonstrating that nitrification was mass transfer rather than kinetically controlled. A reduction in nitrification occurred when glucose (500 mg l-1) was added to the feed due to the growth of a heterotrophic population. The adsorbed biomass reactors lost 35% of nitrification compared to only 7% with PVA, and it appears that the colonisation of PVA by heterotrophs is more difficult than for Linpor and Kaldnes. Respiration rates for all particles increased with time in the reactors, and nitrifiers immobilised in PVA retained approximately 40% of their viability after immobilisation. Volumetric nitrification rates were generally higher for the PVA reactor than for Linpor and Kaldnes, and were: suspended biomass reactor: 0.36; Linpor: 0.57; Kaldnes: 0.53 and PVA: 0.70 kg N m-3-reactor d-1 for a 25% reactor fill. These equate to 2.28, 4.24 and 3.97 g N m-2-media d-1 for Linpor, Kaldnes and PVA respectively, hence other reactor fill rates for Kaldnes warrant further investigation. However, the PVA particles with the highest nitrification rates under all conditions showed promise as an immobilisation medium, and are amenable to further optimisation for the nitrification of high-strength ammonia wastewaters.     

Biodegradation of agricultural herbicides in sequencing batch reactors under aerobic or anaerobic conditions

This study investigated the biodegradability of the herbicides isoproturon and 2,4-dichlorophenoxyacetic acid (2,4-D) in sequencing batch reactors (SBRs). Two laboratory-scale (2L liquid volume) SBRs were employed: one reactor performing under aerobic and the other under anaerobic conditions. The aerobic SBR was operated at an ambient temperature (22+/-2 degrees C), while the anaerobic SBR was run in the lower mesophilic range (30+/-2 degrees C). Each bioreactor was seeded with a 3:1 mixture (by weight) of fresh sludge and biomass that had been previously exposed to both herbicides. The effect of herbicide concentration on either treatment process was explored at a hydraulic retention time (HRT) of 48 h, using glucose as a supplemental carbon substrate. Although no isoproturon degradation was observed in either system during the study, complete 2,4-D removal occurred after an acclimation period of approximately 30 d (aerobic SBR) and 70 d (anaerobic SBR). The aerobic reactor achieved complete 2,4-D utilization at feed concentrations up to 500 mg/L. A further increase to 700 mg/L, however, proved to be inhibitory since 2,4-D biodegradation was negligible. On the other hand, the anaerobic SBR was able to degrade 120 mg/L of 2,4-D, which corresponds to 40% of the maximum feed concentration applied. Moreover, glucose was consumed first throughout the experiment in a sequential utilization pattern relating to 2,4-D, with biodegradation of both substrates following closely first-order kinetics.     

Influence of process loading intensity on sludge clarification and thickening characteristics

The separation of microbial mass in an activated sludge system depends on the formation of large settleable sludge flocs. Microbial cells agglomerate into small clumps and subsequently into sludge flocs by a natural bioflocculation process. The resulting biological mass may exist as individual cells, small aggregates, or large flocs in the aeration basin effluent. In the secondary clarifier, only large flocs are separated by sedimentation while individual cells and small aggregates are washed out as effluent suspended solids.

In the present study, the separation of sludge by sedimentation was shown to be closely related to the surface roughness of the resulting flocs. Normal sludges were observed to have relatively smooth surfaces while filamentous and zoogloeal bulking sludges were noted to have rough and diffused surfaces. The filamentous growths and the non-filamentous protuberances on the bulking sludge surface serve to increase the frictional drag between the moving solid and the liquid. This contributes to a lower settling velocity and associated dewatering rate of bulking sludges.

Conversely, the quantity of non-settleable solids was shown to be related to the quantity of extracellular biopolymers excreted by the microorganisms. The quantity of biopolymers produced decreased for higher Process Loading Intensity or lower sludge age levels thereby resulting in poor aggregation of cells into clumps and subsequently into flocs. This contributes to low clarification efficiencies of the activated sludge process.