Sludge Treatment System Research Articles

Treatment of sewage sludge using electrokinetic geosynthetics

The treatment and disposal of sewage sludge is one of the most problematical issues affecting wastewater treatment in the developed world. The traditional outlets for sewage sludge are to spread it on agricultural land, or to form a cake for deposit to landfill or incineration. In order to create a sludge cake, water must be removed. Existing dewatering technology based on pressure can only remove a very limited amount of this water because of the way in which water is bound to the sludge particles or flocs. Several researchers have shown that electrokinetic dewatering of sludge is more efficient than conventional hydraulically driven methods. This involves the application of a dc voltage across the sludge, driving water under an electrical gradient from positive (anode) electrode to negative (cathode) electrode. However, there have been several reasons why this technique has not been adopted in practice, not least because the, normally metallic, anode rapidly dissolves due to the acidic environment created by the electrolysis of water. This paper will describe experimentation using electrokinetic geosynthetics (EKG): polymer-based materials containing conducting elements. These have been used to minimise the problem of electrode corrosion and create a sludge treatment system that can produce dry solids contents in excess of 30%. It will suggest different options for the treatment of sludges both in situ in sludge lagoons and windrows, and ex situ as a treatment process.

도장부스 수조 내 슬러지 처리 시스템의 개발

A sludge treatment system for introducing the waste water mixture with paint sludge from the water bath of paint booth and for recycling cleaned water to paint booth after sludge separation is developed. Floated sludge is introduced from the water surface in the bath by using floating-skimmer, and is conveyed by pump to the centrifugal separator where sludge and cleaned water are separated. From the operation results of the present sludge treatment systems applied in actual paint booths, paint sludge can be separated automatically and effectively from water bath with its water content of 60-70%, and sludge-free clean water is returned to paint booth.

TESTING OF TWO ULTRASONIC SLUDGE TREATMENT SYSTEMS FOR CONDITIONING OF THICKENED WASTE ACTIVATED SLUDGE

TESTING OF TWO ULTRASONIC SLUDGE TREATMENT SYSTEMS FOR CONDITIONING OF THICKENED WASTE ACTIVATED SLUDGEThickened waste activated sludge (TWAS) from biological treatment processes is typically less amenable to anaerobic digestion than primary sludge. Recently, ultrasonic conditioning of TWAS has been receiving attention as a way to enhance gas production and volatile solids (VS) destruction. TWAS is composed of cellular material, extracellular polymeric substances (EPS) and some inert materials. The...Author(s)Dan GaryJames BarryRobert MortonChi-Chung TangRobert HorvathJames F. StahlSourceProceedings of the Water Environment FederationSubjectSession 34: Residuals & Biosolids Management: Anaerobic Digestion PretreatmentDocument typeConference PaperPublisherWater Environment FederationPrint publication date Jan, 2005ISSN1938-6478SICI1938-6478(20050101)2005:13L.2803;1-DOI10.2175/193864705783865541Volume / Issue2005 / 13Content sourceWEFTECFirst / last page(s)2803 - 2817Copyright2005Word count227

Implicating the glutathione-gated potassium efflux system as a cause of electrophile-induced activated sludge deflocculation.

The glutathione-gated K(+) efflux (GGKE) system represents a protective microbial stress response that is activated by electrophilic or thiol-reactive stressors. It was hypothesized that efflux of cytoplasmic K(+) occurs in activated sludge communities in response to shock loads of industrially relevant electrophilic chemicals and results in significant deflocculation. Novosphingobium capsulatum, a bacterium consistent with others found in activated sludge treatment systems, responded to electrophilic thiol reactants with rapid efflux of up to 80% of its cytoplasmic K(+) pool. Furthermore, N. capsulatum and activated sludge cultures exhibited dynamic efflux-uptake-efflux responses very similar to those observed by others in Escherichia coli K-12 exposed to the electrophilic stressors N-ethylmaleimide and 1-chloro-2,4-dinitrobenzene and the reducing agent dithiothreitol. Fluorescent LIVE/DEAD stains were used to show that cell lysis was not the cause of electrophile-induced K(+) efflux. Nigericin was used to artificially stimulate K(+) efflux from N. capsulatum and activated sludge cultures as a comparison to electrophile-induced K(+) efflux and showed that cytoplasmic K(+) efflux by both means corresponded with activated sludge deflocculation. These results parallel those of previous studies with pure cultures in which GGKE was shown to cause cytoplasmic K(+) efflux and implicate the GGKE system as a probable causal mechanism for electrophile-induced, activated sludge deflocculation. Calculations support the notion that shock loads of electrophilic chemicals result in very high K(+) concentrations within the activated sludge floc structure, and these K(+) levels are comparable to that which caused deflocculation by external (nonphysiological) KCl addition.

Electrochemical Inactivation of Triclosan with Boron Doped Diamond Film Electrodes

This research investigated an electrochemical method for inactivating contaminated stockpiles of the biocidal agent, triclosan. The goal of the electrolysis was to produce products that were amenable to treatment in conventional activated sludge treatment systems. Triclosan oxidation in electrochemical cells with boron doped diamond (BDD) film anodes was investigated in aqueous solutions at a pH value of 12. Chronoamperometry experiments showed that direct oxidation of triclosan occurred at potentials below those for H2O, Cl-, or OH- oxidation. Measurable rates of triclosan oxidation began at potentials above 0.4 V with respect to the standard hydrogen electrode (SHE), while potentials of 0.5, 1.3, and 1.8 V were required to obtain measurable oxidation rates of H2O, Cl-, and OH-, respectively. At anode potentials below 2 V, the dominant electrode reaction involved direct triclosan oxidation, while indirect oxidation was the dominant pathway at higher potentials. However, cyclic voltammetry experiments showed that direct oxidation of triclosan resulted in the formation of a passivating film on the electrode that could only be removed by oxidation at potentials above 3 V. Direct triclosan oxidation showed a very weak potential dependence, suggesting that its oxidation was limited by chemical dependent factors rather than by an outer-sphere electron transfer reaction. Organic triclosan oxidation products consisted primarily of chlorinated acetic acids and chlorinated phenolic compounds. Although the byproducts of triclosan oxidation became increasingly less reactive with increasing electrolysis time, triclosan could be completely oxidized to CO2 at current densities above 2 mA/cm2. Microtox testing indicated that residual triclosan accounted for nearly all the toxicity in the treated water, despite the fact that chlorinated byproduct concentrations were significantly higher than those of triclosan itself.

Distribution of estrogens, 17β-estradiol and estrone, in Canadian municipal wastewater treatment plants

The distribution of female hormones, 17β-estradiol and estrone, was determined in effluents of 18 selected municipal treatment plants across Canada. Replicate 24-h composite samples were collected from the influent and final effluent of each treatment plant, and the removal efficiency compared to the operational characteristics of the plants. In conventional activated sludge and lagoon treatment systems, the mean concentrations of 17β-estradiol and estrone in influent were 15.6 ng/l (range 2.4–26 ng/l) and 49 ng/l (19–78 ng/l). In final effluents, the mean concentrations of both 17β-estradiol and estrone were reduced to 1.8 ng/l (0.2–14.7 ng/l) and 17 ng/l (1–96 ng/l), respectively. 17β-estradiol was removed effectively, >75% and as high as 98%, in most of the conventional mechanical treatment systems with secondary treatment. The removal of estrone was much more complex with removal varying from 98% to situations where the concentrations in the effluent were elevated above that detected in the influent. The estrogenicity, measured using a transfected estrogen receptor in yeast (YES) assay, was also variable, ranging from high removal to elevations of estrogenicity in final effluent. Although the apparent removals were not statistically correlated with either hydraulic (HRT) or solid (SRT) retention times, plants or lagoons with high SRT were very effective at reducing the levels of hormones. Well-operated plants that achieved nitrification also tended to have higher removal of hormones than those that did not nitrify. Laboratory aerobic reactor experiments confirmed the rapid removal of 17β-estradiol, estrone, and estrogenicity when exposed to sewage slurries.

Upgrading of Existing Sludge Treatment Processes for Phosphorus Management Serving A EBPR WWTP

For a large-scale wastewater treatment plant to comply with phosphorus consents using enhanced biological phosphorus removal processes, its sludge and liquor treatment processes need to be carefully upgraded. In this case study, the wastewater treatment plant of interest has three different types of sludge treated by two different and independent sludge treatment processes. The task of upgrading the sludge treatment systems to serve an EBPR process while satisfying other regulatory and operational constraints in a cost effective way presents an interesting challenge. A range of process options was investigated to include P-rich surplus activated sludge treatment, raw sludge treatment, and sludge liquors treatment. Sludge pre-liming, i. e. to introduce lime slurry into raw liquid sludge before the dewatering stage, was studied in benchscale and full-scale trials for phosphorus precipitation and pathogen reduction. It was applied to the mixture of surplus activated sludge and imported sludge. The results showed that a complete phosphorus precipitation was achieved at above pH 9 with lime addition of 7% (w/w as calcium hydroxide to sludge dry weight). A satisfactory 2-log pathogen reduction was consistently achieved at above pH 11 with lime addition of 14% (w/w). The process significantly simplified the potential upgrading work for sludge and liquor treatment, compared to other alternatives.

Elevated precursor 16S rRNA levels suggest the presence of growth inhibitors in wastewater

Conventional activated sludge systems require bacteria to grow to avoid washout through decay and routine solids wasting. Recently we developed a procedure targeting precursor 16S ribosomal RNA to measure the in situ growth activity of phylogenetically defined microbial populations, and this procedure was used to study the growth of bacteria in activated sludge systems. The current study significantly expands this previous work by quantifying levels of precursor 16S ribosomal RNA within individual cells of pure cultures of bacteria exposed to various culture conditions. Initially, three ranges (Type I, Type II, and Type III) of precursor 16S ribosomal RNA levels were defined by whole cell fluorescence in situ hybridization of a pure culture of Acinetobacter calcoaceticusT prepared in three culture conditions. Low levels of precursor 16S ribosomal RNA (Type I) corresponded to a stationary phase culture prepared overnight in Luria-Bertani medium. Intermediate levels of precursor 16S ribosomal RNA (Type II) corresponded to a culture transferred into fresh Luria-Bertani medium, and high levels of precursor 16S ribosomal RNA (Type III) corresponded to a culture treated with the growth inhibiting antibiotic chloramphenicol. Subsequently, the abundance of individual cells of each Type were measured in four different pure cultures after exposure to 0.45-microm filtered primary effluent collected from four different conventional activated sludge treatment plants in Cincinnati, OH, USA. Individual cells of each Type were observed in the culture of A. calcoaceticusT exposed to each of the four primary effluents. Only Type I cells were observed in cultures of A. johnsoniiT, A. johnsonii strain 210a, and Escherichia coliT exposed to each of the four primary effluents. These results suggest that the growth of A. calcoaceticusT was inhibited by an unidentified component of filtered primary effluent present in each of the four wastewaters; whereas the growth of A. johnsoniiT, A. johnsonii strain 210a, and E. coliT were not inhibited. These results have significance for understanding the growth of phylogenetically defined microbial populations within activated sludge treatment systems. If the pattern of elevated p16S rRNA levels observed in A. calcoaceticusT is prevalent in many microbial populations in activated sludge systems, this may have implications for preventing washout of critical microbial populations that may be experiencing growth inhibition.

Identification and quantification of bisphenol A by gas chromatography and mass spectrometry in a lab-scale dual membrane system.

Endocrine disruptor contamination is an emerging issue of concern in the field of water quality engineering. In this study, a lab-scale microfiltration (MF) and reverse osmosis (RO) based water reclamation system was set up to monitor and evaluate the removal of bisphenol A (BPA), which is a known oestrogenic compound. The identification and quantification of BPA were performed by using gas chromatography coupled with mass spectrometry. It was noted that the detection method used in this study was able to achieve an average recovery ranging from 88.2 to 94.1% of BPA with standard deviations of less than 10% in different spiked samples. The detection limit of the analytical protocol was determined at 20 ng L(-1). Based on the analytical protocol, it was noted that a low level of BPA (1.18-3.04 microg L(-1)) could be detected in feed water (effluent of an activated sludge treatment system) to the dual membrane water reclamation system. The results obtained suggested that BPA could be easily chlorinated by sodium hypochlorite with a dosage of 4 to 5 mg L(-1) and a contact time of 1 to 2 min. In this lab-scale study, a satisfactory removal of BPA was readily obtained by RO and BPA was abated to an undetectable level in the product water. It was noted that the RO rejection characteristic of BPA was not sensitive to the variations in raw feed water characteristics experienced in this study. In addition, it was noted that BPA concentration present in raw feed water did not exert any significant impact on RO performance in terms of BPA rejection. The results of this study demonstrated that membrane technology could be effectively used for BPA removal.

Occurrence and removal of plant sterols in pulp and paper mill effluents

Pulp and paper mill effluents (PPMEs) are believed to contain some naturally occurring endocrine disrupting chemicals (EDCs) such as phytosterols or plant sterols, which may be related to a variety of physiological and morphological abnormalities in fish inhabiting receiving waters. Analyses of effluent streams from two Canadian pulp mills confirmed the presence of five different phytosterols in PPMEs. Sterol contents of biologically treated as well as untreated effluents are presented. ββ-Sitosterol, ββ-sitostanol (or stigmastanol), and campesterol formed the major fraction of plant sterols in PPMEs, accounting for 70 % or more of the total phytosterol content measured. Sterols entering the activated sludge treatment systems were generally removed from the effluents with average removal efficiencies of 72 and 66% for the two mills surveyed. Sorption and biodegradation may be responsible for the observed removal of plant sterols across the biological wastewater treatment systems. Key words: ββ-sitosterol, stigmastanol, campesterol, removal and mass flow of phytosterols or plant sterols, hormone disruptors, pulp mill effluents, activated sludge, industrial wastewater treatment.

Characterization and dewaterability of raw and stabilized sludge using different treatment methods

A comparison of the characteristics and stabilization potential of the four most used sludge treatment systems in Mexico was made. A pilot plant constituted by separate systems for anaerobic and aerobic digestion, lime stabilization, conditioning and dewatering, was built and operated during four months in one of the biological wastewater treatment plants in Acapulco, Mexico. Composting of sludge was also made. An aerobic static pile was built using bulking materials available in the region. A turbine centrifuge was used for dewatering the stabilized sludge and results showed good performance of the device. The main problem for the beneficial use of treated sludge was its pathogenicity. The composting process allowed us to obtain a product with approximately 20 fecal coliform density (MPN/g); with lime stabilization, the sludge produced had a fecal coliform density of 2 MPN/g. From these results, it is concluded that both the composting process and the alkaline stabilization with lime produce a well stabilized sludge, bacteriologically safe that accomplishes the requirements for its use on soil without restrictions. Related to parasitological removal, the best helminth egg removals were obtained also using these two processes. Ascaris sp. densities in raw sludge (309-430 eggs/g) were reduced to a final density of 3-14 eggs/g in the aerobic composting process and to 4-18 eggs/g in the lime stabilized sludge. Removal is not high enough to reach the recommended level for unrestricted use of stabilized sludge.

Performance evaluation of an activated sludge system for removing petroleum hydrocarbons from oilfield produced water

During crude oil and natural gas exploration and production, large volumes of petroleum hydrocarbon containing ‘produced water’ are concurrently recovered. Produced water is the largest volume waste stream in the exploration and production processes of oil and gas. During onshore oil recovery, the petroleum hydrocarbon/water mixture is processed through various separation units where the water extract is either discharged into above ground storage facilities, or re-injected into a subsurface formation as a permanent disposal/secondary recovery process. In offshore or coastal oil recovery operations, produced water is either discharged through shore side outfalls or coastal rim releases (within 4 miles from shore). In many cases, current disposal practices such as these have caused severe environmental petroleum hydrocarbon contamination to surface, ground and coastal waterways. Therefore, a research project was conducted to evaluate the performance of a field (continuous-flow) activated sludge treatment system for removing petroleum hydrocarbons from Southwestern US oilfield generated produced water. Petroleum hydrocarbon removal efficiencies were determined using gas chromatography/mass spectrometry (GC/MS) and total petroleum hydrocarbon (TPH) analysis. The activated sludge treatment unit maintained a TPH removal efficiency of 98–99% at a solids retention time (SRT) of 20 days and a mixed liquor suspended solids concentration (MLSS) of 730 mg/l. Incurred energy costs were $0.17 per barrel (or $0.004 per gallon) of produced water treated.

Benchmarking nitrogen removal suspended-carrier biofilm systems using dynamic simulation

We are witnessing an enormous growth in biological nitrogen removal from wastewater. It presents specific challenges beyond traditional COD (carbon) removal. A possibility for optimised process design is the use of biomass-supporting media. In this paper, attached growth processes (AGP) are evaluated using dynamic simulations. The advantages of these systems that were qualitatively described elsewhere, are validated quantitatively based on a simulation benchmark for activated sludge treatment systems. This simulation benchmark is extended with a biofilm model that allows for fast and accurate simulation of the conversion of different substrates in a biofilm. The economic feasibility of this system is evaluated using the data generated with the benchmark simulations. Capital savings due to volume reduction and reduced sludge production are weighed out against increased aeration costs. In this evaluation, effluent quality is integrated as well.

Oligonucleotide probe hybridization and modeling results suggest that populations consuming readily degradable substrate have high cellular RNA levels

Analyses based on ribosomal RNA (rRNA)-targeted hybridization performed in our laboratory identified two types of bacterial populations: a population with a high RNA level per biomass and a population with a low level of RNA per biomass. To extend these descriptions, the diurnal dynamics of the RNA pool were monitored by rRNA-targeted oligonucleotide probe membrane hybridization. Under the typical diurnal variation in COD loading rate experienced by municipal wastewater treatment plants, the RNA level of the bacterial population with a high level of RNA per biomass varied with changes in the COD loading rate. Under the same conditions, the RNA level of the population with low RNA level per biomass remained constant. A structured biomass model was developed to describe these data. Substrate COD was divided into a readily biodegradable and a slowly biodegradable COD fraction. It was assumed that two specialized populations coexist in municipal activated sludge treatment systems. One population consumes readily degradable COD and the other consumes slowly degradable COD. According to model simulations, the population consuming readily degradable COD has a high level of RNA per biomass under variable substrate concentrations. Comparatively, the population consuming slowly degradable COD has a low level of RNA level per biomass. Furthermore, model simulations reproduced the two diurnal RNA profiles observed in a full-scale municipal activated sludge system. Therefore, we suggest that two populations can be distinguished in municipal activated sludge systems: a population consuming readily degradable substrate and a population consuming slowly degradable substrate.

Collision frequencies of microbial aggregates with small particles by differential sedimentation.

Collision and coagulation rates between microbial aggregates and small particles were measured for individual aggregates (1.0-2.5 mm) that settled through a suspension of fluorescent yellow-green (YG) particles (2.83 microm) placed in a settling column. The microbial aggregates, with an average fractal dimension of 2.26, were generated in a lab-scale sequencing batch reactor (SBR) and also collected from a full-scale activated sludge (AS) treatment system. As calculated from comparisons between the settling velocities observed and those predicted by Stokes' law for impermeable particles, the average fluid collection efficiencies were 0.08 for the SBR aggregates and 0.14 for the AS flocs, which were much lower than those previously reported for nonbiological aggregates of latex microspheres. The collision frequency functions between microbial aggregates and small YG particles were 2 orders of magnitude lower than predicted by the rectilinear model but 1 order of magnitude greater than predicted by a curvilinear model. The overall scavenging efficiencies of suspended particles by the falling microbial aggregates compared well with those observed for the nonbiological aggregates, while the particle removal efficiencies from the flow internal to the microbial aggregates were 1 order of magnitude higher than those of the nonbiological aggregates. It is argued that the permeability of microbial aggregates could be reduced by exopolymeric material clogging the pores within the aggregates. The internal permeation through a bioaggregate thus may not be significant enough to be included in the calculation of its settling velocity; however, the intra-aggregate flow cannot be simply neglected where coagulation is concerned. Streamlines still can penetrate the interior of microbial aggregates, allowing greater collision frequencies with other particles than predicted by the curvilinear model. The narrow and convoluted internal flow passages resulting from the collection of extracellular polymeric substances may also contribute to the higher interior particle removal efficiencies of microbial aggregates than those of more permeable, nonbiological aggregates.

Reed beds: constructed wetlands for municipal wastewater treatment plant sludge dewatering

Reed beds are an alternative technology wastewater treatment system that mimic the biogeochemical processes inherent in natural wetlands. The purpose of this project was to determine the effectiveness of a reed bed sludge treatment system (RBSTS) in southern New England after a six-year period of operation by examining the concentrations of selected metals in the reed bed sludge biomass and by determining the fate of solids and selected nutrients. Parameters assessed in both the reed bed influent and effluent: total suspended solids, biochemical oxygen demand, nitrate-nitrogen and total phosphorus. In addition, the following metals were studied in the reed bed influent, effluent and Phragmites plant tissue and the sludge core biomass: boron, cadmium, chromium, copper, iron, lead, manganese, molybdenum, nickel, and zinc. The removal efficiencies for sludge dewatering, total suspended solids and biochemical oxygen demand were all over 90%. Nitrate and total phosphorus removal rates were 90% and 80% respectively. Overall metals removal efficient was 87%. Copper was the only metal in the sludge biomass that exceeded the standards set by the Massachusetts Department of Environmental Protection for land disposal of sludge. The highest metal concentrations, for the most part, tended to be in the lower tier of the sludge profile. The exception was boron, which was more concentrated in the middle tier of the sludge profile. The data and results presented in this paper support the notion that reed bed sludge treatment systems and the use of reed beds provide an efficient and cost effective alternative for municipal sludge treatment.

The applicability of the API 20E and API Rapid NFT systems for the identification of bacteria from activated sludge

The purpose of this study was to test the applicability of the API 20E and API Rapid NFT systems for the identification of some predominant gram-negative and gram-positive bacteria isolated from lab-scale activated sludge treatment systems. In this study, one lab-scale sequencing batch reactor (SBR) and one lab-scale continuous-flow stirred tank reactor (CFSTR) were setup. After both reactors had reached equilibrium, many pure cultures isolated from the activated sludge in both systems were obtained and many morphological, biochemical, physiological tests were conducted to identify each pure culture. The API 20E system is a standardized, miniaturized version of conventional procedures for rapid identification of Enterobacteriaceae and other gram-negative bacteria, and the Rapid NFT kit is used for the identification of the gram-negative, non-fermentative bacteria. Also, a Phillips 300 Transmission Electron Microscope and a Phillips 301 Transmission Electron Microscope were applied to further verify the identification of some genera. According to the results of this study, it has been concluded that some commercial products, such as API 20E system and API Rapid NFT system, can be applied for the identification of microorganisms only at the genus level. Many other additional morphological, biochemical, and physiological tests are always needed to obtain the exact identification of each microorganism at the species level. More advanced technologies such as 16S rRNA may be necessary, however, for a rapid identification of the total bacterial population. In this study, it has also been found that Brevibacterium acetylicum and Pseudomonas vesicularis are two of the most dominant species in the activated sludge of CFSTR system. Gram-positive bacteria such as members of the genus Arthrobacter have shown to be very significant and predominant in the SBR system.

群馬県の下水処理場と河川における非イオン界面活性剤およびその分解生成物の挙動について

We examined the behavior of nonionic surfactants and their degradation products in two municipal sewage treatment plants with anaerobic-aerobic activated sludge treatment system and seven sampling points in Tone River including tributaries in Gunma prefecture, by LC-MS.In the sewage treatment plants, Nonylphenol ethoxylates (NPE) and alcohol ethoxylates (AE) were removed more than 80% and 90%, respectively, after primary sedimentation tank and anaerobic treatment process. NPE, AE, and nonylphenol (NP) were reduced by 99%, 99.9% and 90-95%, respectively, throughout the treatment. In contrast, nonylphenoxy carboxylic acids (NPEC) rather increased in aeration tank, and were not reduced throughout the treatment. The lengths of ethoxylates of NPE were found shortened after aerobic activated sludge treatment. From these results, NPEC was supposed to generate during degradation of NPE. Since NPE (1), NP and NPEC were detected in the extract of activated sludge, certain portion was found adsorbed into the activated sludge.NPE, AE, NP and NPEC were detected at 0.1-16.4μg·l-1, 0.3-12.8μg·l-1, <0.08-0.2μg·l-1 and <0.008-3.2μg·l-1, respectively, in river waters. OP was detected <0.08μg·l-1 in all points. Concentrations of NPE and AE were comparably higher in downstream, in tributaries and in winter.

Optimal policies for activated sludge treatment systems with multi effluent stream generation

Most industrial processes generate liquid waste, which requires treatment prior to disposal. These processes are divided into sectors that generate effluents with time dependent characteristics. Each sector sends the effluent to wastewater treatment plants through pumping-stations. In general, activated sludge is the most suitable treatment and consists of equalization, aeration and settling tanks. During the treatment, there is an increase in the mass of microorganisms, which needs to be removed. Sludge removal represents the major operating costs for wastewater treatment plants. The objective of this work is to propose an optimization model to minimize sludge generation using a superstructure in which the streams from pumping-stations can be sent to the equalization tank. In addition, the aeration tank is divided into cells that can be fed in series and parallel. The model relies on mass balances, kinetic equations, and the resulting Nonlinear Programming problem generates the best operational strategy for the system feed streams with a high substrate removal. Reductions of up to 30 % can be achieved with the proposed strategy maintened BOD efficiency removal upper than 98 %.

An assessment of sewage sludge stability with a specific oxygen utilization rate (SOUR) test method

Sewage sludge treatment systems are intended to stabilize the sludge so that its disposal or reuse can be environmentally acceptable. However, stabilized sludges may still contain residual biodegradable organic matter that can be environmentally a nuisance. This paper presents a specific oxygen utilization rate [SOUR, mgO2/(gVSS·h)] batch reactor test method to quantify the residual biodegradable organic matter content of the treated (stabilized) sewage sludges. The results of the study, in which 37 SOUR batch tests were done on 10 different sewage sludges, show that the SOUR, when determined over a prolonged period (4 to 5 days), gives an indirect measure of sludge stability defined as the % soluble and particulate residual biodegradable organics in the sludge; to determine % sludge stability, it is necessary to simulate the experimental SOUR data by means of a general activated sludge model. The % sludge stability obtained for the 10 different sludges, taking due account of their soluble and particulate constituents, was consistent with that expected from the stabilization treatment systems to which the sludges were subjected.