Excess Sludge Production Research Articles

Structural Analysis and Dewatering Characteristics of Waste Sludge from WWTP MBR

A pilot‐scale UF membrane bioreactor (MBR) of 1 m3/day capacity was set up in an industrial wastewater treatment plant to evaluate its performance. This study mainly focused on testing the dewaterability and structural analysis of MBR sludge. MBR had 14% reduction of excess sludge production in relative to the conventional activated sludge process (CAS sludge). For dewatering, MBR sludge had comparable dewaterability with the CAS sludge but required nearly 20% less flocculant to reach the highest filterability χ and lowest specific filtration resistance (SRF). This could reduce the cost for running the dewatering facilities and final disposal. Meanwhile the chemical and morphological analyses on MBR sludge exhibited lower EPS (exocellular polymeric substances) content, slightly smaller flocs and more compact morphology. Additionally, to estimate the appropriate polyelectrolyte dose prior to dewatering, we measured the hysteresis loop area of the sludge rheogram (shear stress vs. shear rate) using a co‐axial cylinder viscometer. For both sludges, the area dramatically increased at some critical flocculant dosage and then plateaued off. The critical dosage, though not optimal, still led to an acceptable dewatering performance for the sludge.

Effect of sludge retention time in sludge holding tank on excess sludge production in the oxic‐settling‐anoxic (OSA) activated sludge process

AbstractThe activated sludge process is a core technology in wastewater treatment plants. Excess sludge produced in the process must be treated and disposed of properly and may account for up to 60% of total plant operating cost. Therefore, it is necessary to develop new biological concepts to minimize excess sludge production. The oxic‐settling‐anoxic process (OSA process), a modified activated sludge process, may produce less excess sludge than the conventional activated sludge process. The effect of sludge retention time in the sludge holding tank of the OSA process on excess sludge yield has been studied. Four pilot‐scale activated sludge systems were employed, one of which was a conventional activated sludge process, and was used as the control system. The other three were OSA systems operated with different sludge retention times (5.5 h, 7.6 h, and 11.5 h) in the sludge holding tank. All systems were operated with synthetic wastewater for 7 months. Results showed that the three OSA processes with 5.5 h, 7.6 h, and 11.5 h sludge retention time reduced the excess sludge by 33%, 23% and 14%, respectively. Compared to the control process, chemical oxygen demand (COD) removal efficiency and effluent NH3–N concentration were not significantly influenced, but total nitrogen (TN) removal efficiency decreased by 0–9%. Total phosphorus (TP) removal efficiency of OSA processes with 7.6 h and 11.5 h sludge retention time increased by 19%. Sludge settleability was excellent in the three OSA processes. No distinct shift in the diversity of the predominant species was found in microbial populations. We conclude that the OSA system could reduce excess sludge production. Results suggest 6–7 h sludge retention time would be optimal. Copyright © 2007 Society of Chemical Industry

Evaluation of Operation of Submerged Aerated Filters in Wastewater Treatment and Excess Sludge Production

The aims of this study were to evaluate the SCOD removal efficiency in submerged biological aerated filters (BAFs) and subsequently to assess the relationship between the specific surface area (Ss) and Excess Sludge Production (ESP) rate in such filters. Accordingly, four filters with different porosity and specific surface (647, 295, 175 and 136 m2 m(-3)) have been loaded with synthetic wastewater based on low fat dry milk powder with COD of 1500 mg L(-1) in different hydraulic retention time (8, 4, 2, 1 and 0.5 h). In this study, it was shown that specific surface increase of the filters initially increases the efficiency of the filters and after a certain value, the filter efficiency remains uncharged or decreases, as in HRT of 8 h the SCOD removal efficiency of filters 1 and 2 (Ss = 647 m2 m(-3) and Ss = 295 m2 m(-3)) were 89.8 and 91% at VOL of 3.195 and 3.727 kg COD m(-3) d(-1), respectively. Also, the results showed that in all the filters, the production rate of suspended sludge increases inversely with the hydraulic retention time and the media with higher porosity (90.5%) produced less suspended sludge (8-57 g m(-3)) despite having lower efficiency in the SCOD removal.

Continuous and sequencing membrane bioreactors applied to food industry effluent treatment

This work focuses on the performances of two immersed membrane bioreactors used for the treatment of easily biodegradable organic matter present in food industry effluents, for the purpose of water reuse. Two reactor functioning modes (continuous and sequencing) were compared in terms of organic carbon removal and of membrane permeability. For each working mode, pollutant removal was very high, treated water quality presented a low COD concentration (< 125 mg x L(-1)), no solids in suspension and low turbidity (< 0.5 NTU). The quality of the treated water (including germ removal) enabled its reuse on site. Moreover, by developing high biomass concentrations in the reactor, excess sludge production remained very low (< 0.1 gVSS x gCOD(-1)). The performances appeared slightly better for the continuous system (lower COD concentration in the effluent, < 50 mg x L(-1), and lower sludge production). In terms of filtration, a distinct difference was observed between continuous and sequencing systems; transmembrane pressure showed a small and constant evolution rate in continuous membrane bioreactor (CMBR) although it appeared more difficult to control in sequencing membrane bioreactor (SMBR) probably due to punctually higher permeate flow rate and modified suspension properties. The rapid evolution of membrane permeability observed in SMBR was such that more frequent chemical cleaning of the membrane system was required.

Potential of activated sludge ozonation

The disposal of sewage sludge and the agricultural use of stabilised sludge are decreasing due to more stringent regulations in Europe. An increasing fraction of sewage sludge must therefore be dewatered, dried, incinerated and the ashes disposed of in landfills. These processes are cost-intensive and also lead to the loss of the valuable phosphate resources incorporated in the sludge ash. The implementation of processes that could reduce excess sludge production and recycle phosphate is therefore recommended. Partial ozonation of the return sludge of an activated sludge system significantly reduces excess sludge production, improves the settling properties of the sludge and reduces bulking and scumming. The solubilised COD will also improve denitrification if the treated sludge is recycled to the anoxic zone. However, ozonation partly kills nitrifiers and could therefore lead to a decrease of the effective solid retention time of the nitrifier, thus reducing the safety of the nitrification. This paper discusses the effect of ozonation on sludge reduction, the operating stability of nitrification, the improvement of denitrification and also presents an energy and cost evaluation.

AEROBIC DEGRADATION OF A NON-IONIC SURFACTANT IN A MEMBRANE BIOREACTOR(MBR)

A membrane bioreactor (MBR) was used to investigate the aerobic degradation of foam active substance - non-ionic surfactant, APG 2000 UP. The surface aeration using the propeller loop reactor (PLR) guaranteed sufficient O₂ for substrate removal and bacteria growth and avoided foam development. Moreover, the cross-flow membrane filtration enabled the separation of the bacteria still loaded with surfactant in the collecting container. The biological degradation of the surfactant with varying hydraulic retention time (HRT) and influent concentration cs0 showed high substrate removal of nearly 95 % at high volumetric loading rates up to 7.4 kgCOD m(-3) d(-1) and at sludge loading rates up to 1.8 kgCOD (kgVSS d)-1 for biomass concentration cB ≒ constant. The increasing cB from 3.4 to 14.5 gL-1 TSS respectively sludge retention time (SRT) from 5.1 to 442 d under complete biomass retention by the membrane filtration resulted in high removal of substrate α > 90 % with reducing excess sludge production.

Effects of chemical sludge disintegration on the performances of wastewater treatment by membrane bioreactor

A new wastewater treatment process combining a membrane bioreactor (MBR) with chemical sludge disintegration was tested in bench scale experiments. In particular, the effects of the disintegration treatment on the excess sludge production in MBR were investigated. Two MBRs were operated. In one reactor, a part of the mixed liquor was treated with NaOH and ozone gas consecutively and was returned to the bioreactor. The flow rate of the sludge disintegration stream was 1.5% of the influent flow rate. During the 200 days of operation, the MLSS level in the bioreactor with the disintegration treatment was maintained relatively constant at the range of 10,000–11,000 mg/L while it increased steadily up to 25,000 mg/L in the absence of the treatment. In the MBR with the sludge disintegration, relatively constant transmembrane pressures (TMPs) could be maintained for more than 6 months while the MBR without disintegration showed an abrupt increase of TMP in the later phase of the operation. In conclusion, a complete control of excess sludge production in the membrane-coupled bioreactor was possible without significant deterioration of the treated water quality and membrane performances.

Performance comparison of a pilot-scale UASB and DHS system and activated sludge process for the treatment of municipal wastewater

This study compares the performance of a pilot-scale combination of UASB and DHS system to that of activated sludge process (ASP) for the treatment of municipal sewage. Both systems were operated in parallel with the same sewage as influent. The study was conducted for more than 300 days, which revealed that organic removal efficiency of UASB + DHS system was comparable to that of ASP. Unfiltered BOD removal by both systems was more than 90%. However, UASB + DHS system outperformed ASP for pathogen removal. In addition, volume of excess sludge production from UASB + DHS was 15 times smaller than that from ASP. Moreover, unlike ASP, there is no requirement of aeration for the operation of UASB + DHS system, which makes it an economical treatment system. Considering the above observations, it was concluded that UASB + DHS system can be a cost-effective and viable option for the treatment of municipal sewage over ASP, especially for low-income countries.

The influence of aerobic sludge retention time on anaerobic co-digestion of dyeing and printing wastewater and sewage sludge

Two pilot-scale activated sludge systems comprising anaerobic baffled reactor (ABR) and aerobic plug flow reactor (PFR) were operated aiming to minimize excess sludge output of the activated sludge process through coupled alkaline hydrolysis and anaerobic digestion. Variations in the effluent total chemical oxygen demand (TCOD) and NH4 +-N concentration proved that the process not only could minimize excess sludge production but also guarantee the effluent TCOD well below the discharging limit (150 mg/l) if the inverse ratio of the aerobic sludge recirculation to anaerobic reactor did not exceed 60% for system A with 10 d aerobic sludge retention time (SRT) and 40% for system B (SRT was 25 d). The sludge activity at aerobic SRT of 25 d was evidently lower than aerobic SRT of 10 d. Those differences of sludge characteristics affected the inverse sludge ratio obviously. Aerobic bacteria after internal and external decay were converted to anoxic or anaerobic biomass. The distinct differences in sludge yield of aerobic and anaerobic/anoxic processes could explain how aerobic SRT decided excess sludge activity which consequently affected anaerobic codigestion of printing and dyeing wastewater and sewage sludge.

Comparison between inhibitor and uncoupler for minimizing excess sludge production of an activated sludge process

In order to study the minimization of excess sludge production, the reduction in the excess sludge production in the presence of an inhibitor and uncoupler was studied in this work. The experimental results show that such an addition could effectively reduce the production of excess sludge. With the energy uncoupling model established in this work, energy uncoupling coefficient (E u) was used to evaluate the reduction in excess sludge production. The energy uncoupling coefficients in the presence of dinitrophenol (dNP), Zn2+, and Cu2+ was 0.75, 0.46, and 0.18, respectively. The analysis demonstrated that energy spilling occurred in the presence of dNP, and that dNP was an effective uncoupler for reducing the production of excess activated sludge without affecting the microbial respiration activity.

Organic matter release in low temperature thermal treatment of biological sludge for reduction of excess sludge production

Thermal treatment applied in association with a biological system allows for a significant reduction in excess sludge production (approximately 50%). In general, heat treatment is described as a sludge disintegration technique. This paper offers a thorough study on the impact of heat treatment, at temperatures below 100 degrees C, on the solubilisation of the sludge COD and its biodegradability. Discontinuous heating experiments were performed on activated and digested sludge. At all temperatures tested the released COD for digested sludge was systematically higher than that for activated sludge (15 and 40%, respectively, at 95 degrees C for 40 min of contact time). For the first 30 min, a 1st order kinetic, with respect to the residual COD, was systematically found. In the range of 40-95 degrees C, digested sludge had a lower activation energy than activated sludge (26 kcal/mol compared to 70-160 kcal/mol). COD solubilisation is thus more positively influenced by temperature in the case of activated sludge. This may be due to the significant difference in the ratio of protein/carbohydrate in digested and activated sludge (1-5 and 0.2-0.7, respectively). The increase in the COD/TKN ratio in the solubilised fraction after thermal treatment of activated sludge suggests a preferential solubilisation of proteins over carbohydrates. Respirometric tests performed on the solubilised COD showed that whatever the sludge origin, only 40-50% of released COD is biodegradable at a conventional hydraulic retention time (i.e., 24 h). Hence, heat treatment would act more through organic matter solubilisation rather than by a biodegradability increase.

Biological sludge solubilisation for reduction of excess sludge production in wastewater treatment process

A novel sludge disintegration system (JFE-SD system) was developed for the reduction of excess sludge production in wastewater treatment plants. Chemical and biological treatments were applied to disintegrate excess sludge. At the first step, to enhance biological disintegration, the sludge was pretreated with alkali. At the second step, the sludge was disintegrated by biological treatment. Many kinds of sludge degrading microorganisms integrated the sludge. The efficiency of the new sludge disintegration system was confirmed in a full-scale experiment. The JFE-SD system reduced excess sludge production by approximately 50% during the experimental period. The quality of effluent was kept at quite a good level. Economic analysis revealed that this system could significantly decrease the excess sludge treatment cost.

Reduction of excess sludge production using mechanical disintegration devices

The usability of mechanical disintegration techniques for the reduction of excess sludge production in the activated sludge process was investigated. Using three different disintegration devices (ultrasonic homogeniser, stirred media mill, high pressure homogeniser) and different operational parameters of the disintegration, the effect of mechanical disintegration on the excess sludge production and on the effluent quality was studied within a continuously operated, laboratory scale wastewater treatment system with pre-denitrification. Depending on the operational conditions and the disintegration device used, a reduction of excess sludge production of up to 70% was achieved. A combination of mechanical disintegration with a membrane bioreactor process with high sludge age is more energy effective concerning reduction of sludge production than with a conventional activated sludge process at lower sludge ages. Depending on the disintegration parameters, the disintegration has no, or only minor, negative effect on the soluble effluent COD and on the COD-removal capacity of the activated sludge process. Nitrogen-removal was slightly deteriorated by the disintegration, whereas the system used was not optimised for nitrogen removal before disintegration was implemented.

Contrasting the Benefits of Primary Clarification versus Prefermentation in Activated Sludge Biological Nutrient Removal Systems

The potential benefits prefermentation can provide to biological nutrient removal are measured and compared to the costs of excess oxygen consumption and sludge production incurred by an activated sludge system that utilizes prefermentation, instead of primary clarification. Prefermentation was found to produce superior performance in regards to enhanced biological phosphorus removal. A lower soluble orthophosphorus effluent value [ 3.2 mg∕L for the prefermented activated sludge (PAS) train versus 4.6 mg∕L for the control train with primary clarification (PCAS)] and a higher percent phosphorus (% P) content of the biomass (9.0% for the PAS train versus 7.8% for the PCAS train) were both found to be statistically significant (P values of 4.26× 10−5 and 0.0082, respectively). In addition statistically significant improvements in denitrification rates and reduced observed yields were observed due to prefermentation. However statistically significant increases in solids inventory and in particular oxygen upta...

A Review and Assessment of Emerging Technologies for the Minimization of Excess Sludge Production in Wastewater Treatment Plants

This paper focuses on the most promising technologies, available for full-scale applications, aimed to the on-site reduction of the excess sludge produced in municipal wastewater treatment plants. New techniques are added to the conventional stages of wastewater treatment, both integrated in the activated sludge bioreactors or applied as pretreatment for the enhancement of anaerobic digestion. A concise review about the alternatives based on physical, chemical or biological mechanisms is described. The present work highlights the efficiency of two such techniques, sonolysis and alkaline thermolysis integrated on the return flow from the secondary settler into the activated sludge bioreactors. The investigation on the effect of sonolysis and alkaline thermolysis on activated sludge samples was carried out by evaluating the COD concentration released in soluble and colloidal form and biodegradability measured by respirometry. The physicochemical treatments of sludge have several advantages (easy management, stability in performances and flexibility), but are associated with high operational costs that often limit the wide-scale applications. The application of hybrid methods, that couple almost two techniques for the enhancement of efficiency with respect to a single one, could optimise the sludge reduction, giving a significant saving in energy consumption for large-scale operations, but further research is needed.

Membrane bioreactor performances: comparison between continuous and sequencing systems

Nowadays, membrane bioreactor performancesare well known in term of quality of pollutantremoval from wastewater including possibility oftreated water reuse. As regards the excess sludgeproduction, the strict levels of environmentaldirective and the limiting disposal options,imposed to develop strategies on the reductionof biomass production in wastewater treatment [1].A type of strategy consists in limiting the amountof substrate supplied to the biomass, i.e. the foodto microorganisms (F/M) ratio. In this substrate-limited condition the allocation of the substrateis preferentially turned to maintenance energyrequirements [2,3]. However, to ensure a capabil-ity of the system to treat acceptable organicvolumetric loadings, development of reactors withhigh biomass concentration is essential. On thisway, fixed cultures and specially membrane bio-reactors (MBR) appears as adequate solutions. Indeed, according to Pirt theory [3], it seemspossible, for a given F/M ratio, to reach asteady state biomass con centration in a continu-ous system without any sludge extraction andconsequently, a MBR system should be able toachieve a stable biomass concentration in thereactor avoiding any excess sludge production.As well Ref. [4] showed that the initial sub-strate/biomass ratio is a critical parameter inbatch experiments: when cell growth is limitedby carbon source, low initial substrate/biomassratios, the bacterial metabolism imposed stor-age compounds synthesis. This phenomenon —synthesis and consumption — leads to biomassconversion yields lower than a biomass replica-tion process which is favourable at highsubstrate/biomass ratios [5]. This result may bean important point when comparing continuousand sequencing bioreactor functioning at thesame organic loading. Indeed a continuousfeeding of the reactor corresponds to a continu-ous low F/M ratio (with adapted MLVSS in thereactor) then a sequencing system will receivemomentously higher F/M ratio during the feedperiod. The aim of this work is to study and comparethe biological and filtration performances of twoimmersed membranes bioreactors, one is operat-ing in a sequencing way (MSBR), the other onein a continuous way (MCBR). The operations

Characteristics of simultaneous organic and nutrient removal in a pilot-scale vertical submerged membrane bioreactor (VSMBR) treating municipal wastewater at various temperatures

A pilot-scale vertical submerged membrane bioreactor (VSMBR) with anoxic and oxic zones in one reactor was operated in an attempt to reduce the problems concerning effective removal of organic matter and nutrients from municipal wastewater. Source water with total chemical oxygen demand (TCOD)/total nitrogen (TN) ratio of 5.5 was treated at various temperatures (13–25 °C) over an interval of about 1 year. As a result, total suspended solid (TSS) and TCOD were removed by 100% and higher than 98%, respectively. Moreover, the average removal efficiencies of TN and total phosphorus (TP) were found to be 74% and 78% at 8 h-hydraulic retention time (HRT) and 60-days sludge retention time (SRT). Under these conditions, the specific removal rates (SRR) of TN and TP were found to be 0.093 kg N m −3 day −1 and 0.008 kg P m −3 day −1, and the daily production of excess sludge (DPES), 0.058 kg TSS day −1.

Technical and Economical Evaluation of a Thermal, and Two Oxidative Techniques for the Reduction of Excess Sludge Production

Municipal wastewater treatment is closely linked with the production of sludge. This production may be reduced by combining a sludge disintegration technique with a conventional biological reactor, such as an activated sludge. The disintegration techniques are based on mechanical, electrical, thermal, thermo-chemical, biological and oxidative treatments. This work tries to give an evaluation of the efficiency and the economics of disintegration techniques that have demonstrated their potential to reduce the excess sludge production when combined to an activated sludge plant fed with a real urban wastewater. A high ESP reduction rate is achievable (more than 40%) by using disintegration technique such as a thermal (95°C), an ozonation or a hydrogen peroxide treatment. These three disintegration techniques were studied in details in our laboratories. The experiments carried out provided the technical data in order to make an assessment of the economics of the three selected combined processes. The ozone appeared to be the most interesting route for sludge reduction but the heating route is also economically competitive with conventional sludge treatment and disposal especially if stringent constraints on the sanitary quality of the sludge are imposed.

Comparison of sludge characteristics and performance of a submerged membrane bioreactor and an activated sludge process at high solids retention time

This work aims to compare biomass structure and performance of a submerged membrane bioreactor (SMBR) and an activated sludge process (ASP) treating the same domestic wastewater. The influence of the separation technique (membrane filtration or settling) and operation at high sludge-retention time (SRT) were investigated. Over the entire range of SRT (10–110 days), the SMBR achieved very good organic removal efficiencies, ranging from 90.8±0.2% to 94.2±1.6% based on total COD (TCOD), whereas those of ASP were between 87.4±1.8% and 90.3±0.8%. The contribution of the membrane in the increase in performance was due to total suspended solid retention and also partly due to retention of proteins and polysaccharides of the sludge supernatant. No significant difference in excess sludge production was observed between the two processes operated at the same SRT, but sludge production in SMBR decreased from 0.31 to 0.13 g VSS g COD −1 as SRT increased from 9 to 110 days. The difference in sludge characteristics and performance was especially pronounced as SRT increased, resulting in deterioration of sludge settleability and effluent quality of the ASP (filamentous bacteria, increase of protein and polysaccharide release). Membrane filtration induced accumulation of soluble and colloidal proteins and polysaccharides which were progressively degraded in the supernatant as the SRT increased. At similar SRT, no significant difference was observed in the amount of extractable exocellular polymeric substances (bound EPS) from ASP and SMBR sludge. However as the SRT increased, the total specific amount of bound EPS in flocs decreased and the ratio proteins/polysaccharides also decreased. Concomitantly, laser diffraction analysis, microscopic observations, turbidity and DSVI measurement showed that the SRT increase induced significant modifications in sludge morphology in SMBR: decrease in floc size, densification of aggregates, and development of non-flocculating organisms.

Operation of a new sewage treatment process with technologies of excess sludge reduction and phosphorus recovery

This paper shows the potential application of a new sewage treatment process with technologies of excess sludge reduction and phosphorus recovery. The process incorporated ozonation for excess sludge reduction and crystallisation process for phosphorus recovery to a conventional anaerobic/oxic (A/O) phosphorus removal process. A lab-scale continuous operation experiment was conducted with the ratio of sludge flow rate to ozonation tank of 1.1% of sewage inflow under 30 to 40 mgO3/gSS of ozone consumption and with sludge wasting ratio of 0.34% (one-fifth of a conventional A/O process). Throughout the operational experiment, a 60% reduction of excess sludge production was achieved in the new process. A biomass concentration of 2300 mg/L was maintained, and the accumulation of inactive biomass was not observed. The new process was estimated to give a phosphorus recovery degree of more than 70% as an advantage of excess sludge reduction. The slight increase in effluent COD was observed, but the process performance was maintained at a satisfactory level. These facts demonstrate an effectiveness of the new process for excess sludge reduction as well as for phosphorus recovery.