Biogas production from cheese whey wastewater: laboratory- and full-scale studies.

A two-phase system for biogas production from cheese whey wastewater (CWW) was designed, set up and operated at laboratory and full scale for a whole cheese production season (8-9 months). The high efficiency and stability of the laboratory-scale system was demonstrated under various organic loading rates (OLRs) reaching 13 g chemical oxygen demand (COD) L(-1)d(-1) and producing up to 9 L L(-1)d(-1) of biogas (approximately 55% in methane). The COD removal was above 95% and the pH was maintained above 6.3 without any chemical addition. The full-scale system was operated at lower OLRs than its normal capacity, following the good response and high stability in disturbances of the laboratory-scale unit.

Sewage pre-concentration for maximum recovery and reuse at decentralized level.

Pre-concentration of municipal wastewater by chemically enhanced primary treatment (CEPT) was studied under controlled laboratory conditions. Both iron and aluminium-based coagulants were examined at gradually increasing concentrations (0.23, 0.35, 0.70 and 1.05 mmol/L). The CEPT sludge generated from different coagulation experiments was digested in batch anaerobic reactors, while the supernatant was tested in a dead-end microfiltration setup. The results of the study show that biogas yield was dramatically decreased (from 0.40 to 0.10 m(3)/kg chemical oxygen demand of influent) with increasing coagulant dose. In contrast, supernatant filterability was improved. Based on the laboratory results, a conceptual design was produced for a community of 2000 inhabitant equivalents (IE), using CEPT technology (at low coagulant dose) with anaerobic digestion of the concentrates. According to this, the capital and operational costs were 0.11 and 0.09 €/m(3), respectively. The biogas generated is used for digester heating and the overall process is energy self-sufficient. At a small-scale and in private applications, CEPT technology is preferably operated at higher coagulant dose, followed by membrane filtration for water reuse. Accordingly, sewage purification and reuse is possible without implementing aerobic biological processes.

Comparison between aerobic and anoxic metabolism of denitrifying-EBPR sludge: effect of biomass poly-hydroxyalkanoates content.

Biomass with denitrifying phosphate uptake ability was tested under sequencing anaerobic-aerobic and anaerobic-anoxic conditions. The initial dose of acetate, under anaerobic conditions varied to achieve different PHA (poly-hydroxyalkanoates) saturation of PAO (polyphosphate accumulating organisms) cells. Increased acetate dosage under anaerobic conditions led to higher phosphate release and increased PHA storage by PAOs and, also, to greater phosphate uptake rates under the following aerobic and/or anoxic conditions. The experimental results also indicated that when organic carbon is limited under anaerobic conditions, more internal glycogen supplementary to polyphosphate cleavage is utilized by the biomass, resulting in less phosphate release and more PHA stored per acetate taken up. In the subsequent aerobic and/or anoxic phase PAOs demonstrate an improved EBPR (enhanced biological phosphorus removal) performance, with regard to PHA consumption per phosphate taken up, for reduced initial biomass PHA content under both aerobic and anoxic conditions. The examination of EBPR biomass under controlled operational conditions, where experimental analysis of the relevant compounds in the bulk phase (PO(4)(3-), NO(3)(-) and/or O(2)) in conjunction with the biomass intracellular products (PHA, glycogen), contributes to an improved understanding of the PAOs metabolic behavior, with regard to organic substrate availability.

Anaerobic waste activated sludge co-digestion with olive mill wastewater.

Co-digestion of waste activated sludge (WAS) with agro-industrial organic wastewaters is a technology that is increasingly being applied in order to produce increased gas yield from the biomass. In this study, the effect of olive mill wastewater (OMW) on the performance of a cascade of two anaerobic continuous stirred tank (CSTR) reactors treating thickened WAS at mesophilic conditions was investigated. The objectives of this work were (a) to evaluate the use of OMW as a co-substrate to improve biogas production, (b) to determine the optimum hydraulic retention time that provides an optimised biodegradation rate or methane production, and (c) to study the system stability after OMW addition in sewage sludge. The biogas production rate at steady state conditions reached 0.73, 0.63, 0.56 and 0.46 l(biogas)/l(reactor)/d for hydraulic retention times (HRTs) of 12.3, 14, 16.4 and 19.7 d. The average removal of soluble chemical oxygen demand (sCOD) ranged between 64 and 72% for organic loading rates between 0.49 and 0.75 g sCOD/l/d. Reduction in the volatile suspended solids ranged between 27 and 30%. In terms of biogas selectivity, values of 0.6 l(biogas)/g tCOD removed and 1.1 l(biogas)/g TVS removed were measured.

Cotton-textile wastewater management: investigating different treatment methods.

The cotton-textile industry consumes significant amounts of water during manufacturing, creating high volumes of wastewater needing treatment. The organic-load concentration of cotton-textile wastewater is equivalent to a medium-strength municipal wastewater; the color of the water, however, remains a significant environmental issue. This research, in cooperation with a cotton-textile manufacturer, investigated different treatment methods and different combinations of methods to identify the most cost-effective approaches to treating textile wastewater. Although activated-sludge is economical, it can only be used as part of an integrated wastewater management system because it cannot decolorize wastewater. Coagulation/flocculation methods are able to decolorize cotton-wastewater; however, this process creates high amounts of wastewater solids, thus significantly increasing total treatment costs. Chemical oxidation is an environmentally friendly technique that can only be used as a polishing step because of high operating costs. Anaerobic digestion in a series of fixed-bed bioreactors with immobilized methanogens using acetic acid as a substrate and a pH-control agent followed by activated-sludge treatment was found to be the most cost-effective and environmentally safe cotton-textile wastewater management approach investigated.

Effect of basic operating parameters on biological phosphorus removal in a continuous-flow anaerobic-anoxic activated sludge system.

A continuous-flow anaerobic-anoxic (A2) activated sludge system was operated for efficient enhanced biological phosphorus removal (EBPR). Because of the system configuration with no aeration zones, phosphorus (P) uptake takes place solely under anoxic conditions with simultaneous denitrification. Basic operating conditions, namely biomass concentration, influent carbon to phosphorus ratio and anaerobic retention time were chosen as variables in order to assess their impact on the system performance. The experimental results indicated that maintenance of biomass concentration above 2,500 mg MLVSS/L resulted in the complete phosphate removal from the influent (i.e. 15 mg PO(4) (3-)-P/L) for a mean hydraulic residence time (HRT) of 15 h. Additionally, by increasing the influent COD/P ratio from 10 to 20 g/g, the system P removal efficiency was improved although the experimental results indicated a possible enhancement of the competition between phosphorus accumulating organisms (PAOs) and other microbial populations without phosphorus uptake ability. Moreover, because of the use of acetate (i.e. easily biodegradable substrate) as the sole carbon source in the system feed, application of anaerobic retention times greater than 2 h resulted in no significant release of additional P in the anaerobic zone and no further amelioration of the system P removal efficiency. The application of anoxic P removal resulted in more than 50% reduction of the organic carbon necessitated for nitrogen and phosphorus removal when compared to a conventional EBPR system incorporating aerobic phosphorus removal.

Process control, energy recovery and cost savings in acetic acid wastewater treatment.

An anaerobic fixed bed loop (AFBL) reactor was applied for treatment of acetic acid (HAc) wastewater. Two pH process control concepts were investigated; auxostatic and chemostatic control. In the auxostatic pH control, feed pump is interrupted when pH falls below a certain pH value in the bioreactor, which results in reactor operation at maximum load. Chemostatic control assures alkaline conditions by setting a certain pH value in the influent, preventing initial reactor acidification. The AFBL reactor treated HAc wastewater at low hydraulic residence time (HRT) (10-12 h), performed at high space time loads (40-45 kg COD/m(3) d) and high space time yield (30-35 kg COD/m(3) d) to achieve high COD (Chemical Oxygen Demand) removal (80%). Material and cost savings were accomplished by utilizing the microbial potential for wastewater neutralization during anaerobic treatment along with application of favourable pH-auxostatic control. NaOH requirement for neutralization was reduced by 75% and HRT was increased up to 20 h. Energy was recovered by applying costless CO(2) contained in the biogas for neutralization of alkaline wastewater. Biogas was enriched in methane by 4 times. This actually brings in more energy profits, since biogas extra heating for CO(2) content during biogas combustion is minimized and usage of other acidifying agents is omitted.

Recovery of reusable water from sewage using aerated flat-sheet membranes.

Continuous developments and advances in membrane technology allow recovering to large extent reusable water from untraditional water sources, such as municipal effluents. In this paper, operational results and preliminary cost-analysis of a microfiltration (MF) process used for raw sewage and secondary effluent polishing are given. The research was conducted with a 22 sandwich-type flat-sheet membrane module (0.45 µm and 4.5 m(2)) employing aeration for fouling control. During raw sewage filtration the majority of the organics were retained (effluent COD < 90 mg/L and SS ~ 0 mg/L). However, the fluxes were significantly lower [~7 L/(m(2) h)] compared to those achieved during secondary effluent filtration [~29 L/(m(2) h)] (at TMP 0.14 bar). In the second case, aeration was found to be suitable mean for fouling control. The total costs (capital and operational) for water recovery from raw sewage and secondary effluent were estimated to be in the order of 2 and 0.75 €/m(3) respectively.

Comparison between UCT type and DPAO biomass phosphorus removal efficiency under aerobic and anoxic conditions.

Two different types of biomass, capable for Enhanced Biological Phosphorus Removal (EBPR), a UCT (University of Cape Town) type and a sludge enriched with DPAOs (Denitrifying Phosphorus Accumulating Organisms) were tested in batch reactors under specific operational and environmental conditions, in order to achieve a direct comparison of their phosphorus removal capability. Three types of batch reactors were operated, Anaerobic/Oxic (AO), Anaerobic/Anoxic (A2) and Anaerobic/Anoxic/Oxic (A2O), under controlled temperature and pH conditions. Maximum anaerobic specific phosphate release, substrate utilization, as well as denitrification and phosphate uptake rates under aerobic and anoxic conditions were determined and compared for the two different microbial populations. Experimental results indicated no significant difference between the anoxic and the aerobic phosphorus (P) uptake rates, respectively for DPAO and UCT sludge. The UCT sludge was also found to achieve anoxic P uptake, however to much less extend compared to the DPAO sludge. It has also been proved that anoxic P uptake seems to negatively affect the total P removal efficiency of this type of sludge, even under following aerobic conditions. Based on these findings, denitrifying phosphorus removal systems are proved comparable to conventional EBPR configurations (UCT), concerning phosphorus removal efficiency, while their operation is accompanied by potential advantages.

Upflow anaerobic clarification tank (UACT) to upgrade existing anaerobic effluents.

The Upflow Anaerobic Clarification Tank (UACT) is of interest as a post-treatment step for high-rate anaerobic wastewater treatment systems. The UACT contributes to extra capture of methane from the raw wastewater. It also significantly facilitates the reuse of the treated water. The efficiency of the UACT process is demonstrated in this study using laboratory-scale reactors. The water produced by the UACT had a low total COD, in most cases below 145 mg/L and a soluble COD below 60 mg/L at surface loading rates between 0.1-0.4 m(3)/(m(2) h) and hydraulic retention times between 5-6 h. The sludge bed of the UACT was characterised by 6-10 times higher substrate affinity (for ethanol, acetate and propionate) compared to the parent UASB and similar specific activity. The results suggest that a UASB followed by the UACT can, upon subsequent application of a rapid filtration, qualify as a total COD removal, no longer necessitating aerobic treatment.

Effect of temperature on anoxic sulfide oxidation and denitrification in the bulk wastewater phase of sewer networks.

Artificial dosage of nitrate in sewer networks is considered as one of the most effective methods for odor and corrosion control. However, there is limited knowledge on the effect of temperature on the transformations that takes place during anoxic conditions. Thus, two groups of batch experiments were conducted to gain insight in the involved processes in bulk phase of a septic municipal wastewater. It can be concluded that sewer denitrification, in bulk phase, can be simplified in three stages. According to the experimental results, nitrate or nitrite is utilized for autotrophic denitrification with sulfide, while heterotrophic utilization is initiated after the completion of anoxic sulfide oxidation. Moreover, temperature is proved to have a significant impact on sewer denitrification kinetic profile, as it determines the extent of temporal nitrite accumulation. The temperature coefficient of each anoxic process, including sulfide oxidation, nitrate utilization and denitrification/nitrite utilization is experimentally calculated and temperature dependent equations are developed, providing the rate of all anoxic processes in bulk phase of sewer wastewater, in any given temperature.

A modified UCT method for biological nutrient removal: configuration and performance.

A pilot-scale prototype activated sludge system is presented, which combines both, the idea of University of Cape Town (UCT) concept and the step denitrification cascade for removal of carbon, nitrogen and phosphorus. The experimental set-up consists of an anaerobic selector and stepwise feeding in subsequent three identical pairs of anoxic and oxic tanks. Raw wastewater with influent flow rates ranging between 48 and 168 l d(-1) was fed to the unit at hydraulic residence times (HRTs) of 5-18 h and was distributed at percentages of 60/25/15%, 40/30/30% and 25/40/35% to the anaerobic selector, 2nd and 3rd anoxic tanks, respectively (influent flow distribution before the anaerobic selector). The results for the entire experimental period showed high removal efficiencies of organic matter of 89% as total chemical oxygen demand removal and 95% removal for biochemical oxygen demand, 90% removal of total Kjeldahl nitrogen and total nitrogen removal through denitrification of 73%, mean phosphorus removal of 67%, as well as excellent settleability. The highest removal efficiency and the optimum performance were recorded at an HRT of about 9h and influent flow rate of 96 l d(-1), in which 60% is distributed to the anaerobic selector, 25% to the second anoxic tank and 15% to the last anoxic tank. Consequently, the plant configuration enhanced removal efficiency, optimized performance, saved energy, formed good settling sludge and provided operational assurance.

Development and implementation of microbial sensors for efficient process control in wastewater treatment plants.

This paper demonstrates the functionality, laboratory testing and field application of a microbial sensor, which can be modified to monitor organic pollution extent, toxicity and over-(under)load of wastewaters both under anaerobic and aerobic conditions. Since nitrification is related to protons formation and the addition of alkaline is necessary for pH control, an aerobic biosensor monitoring Na(2)CO(3) consumption was developed and practically implemented to control the nitrification process. As CO(2) is the respiration product from aerobic degradation which can be correlated to the organic pollution extent, the previous biosensor was modified to monitor and measure the online toxicity and BOD/COD. Under anaerobic conditions, the online measurement of NaOH consumption and biogas production allowed the detection of toxicity incidents and over-(under)load in the influent. Such toximeters get in contact with the wastewater the earliest possible, providing sufficient time for protection of sensitive biological wastewater treatment processes and for the implementation of control and management strategies.

Absorption of carbon dioxide by raw and treated dye-bath effluents.

A bench-scale experimental apparatus, consisting of a glass column randomly packed with cylindrical glass rings, was utilized for the removal of carbon dioxide (CO(2)) by chemical absorption. Various liquid solvents were used, in a batch mode, circulating between the column and a reservoir vessel, and contacting the gas in counter current flow. The absorptive capacity of the liquid solvents and the absorption kinetics were studied by obtaining the respective 'breakthrough curves'. Aqueous solutions of sodium hydroxide (NaOH) were initially utilized in order to demonstrate the experimental apparatus function. Dye-bath effluent proved highly effective due to its high alkalinity and the capacity of the hydrolyzed "azo-reactive" dyes to react with CO(2). Decolorized dye-bath effluent (treated with FeSO(4)) can also be used as a chemical solvent for the absorption of CO(2); however, its absorptive capacity is much lower compared to the raw wastewater. The above technique is also a very effective method to neutralize industrial effluents.

Decoloration of textile wastewater by means of a fluidized-bed loop reactor and immobilized anaerobic bacteria.

Textile wastewater was treated by means of a fluidized-bed loop reactor and immobilized anaerobic bacteria. The main target of this treatment was decoloration of the wastewater and transformation of the non-biodegradable azo-reactive dyes to the degradable, under aerobic biological conditions, aromatic amines. Special porous beads (Siran) were utilized as the microbial carriers. Acetic acid solution, enriched with nutrients and trace elements, served both as a pH-regulator and as an external substrate for the growth of methanogenic bacteria. The above technique was firstly applied on synthetic wastewater (an aqueous solution of a mixture of different azo-reactive dyes). Hydraulic residence time was gradually decreased from 24 to 6 h over a period of 3 months. Full decoloration of the wastewater could be achieved even at such a low hydraulic residence time (6 h), while methane-rich biogas was also produced. The same technique was then applied on real textile wastewater with excellent results (full decoloration at a hydraulic residence time of 6 h). Furthermore, the effluent proved to be highly biodegradable by aerobic microbes (activated-sludge). Thus, the above-described anaerobic/aerobic biological technique seems to be a very attractive method for treating textile wastewater since it is cost-effective and environment-friendly.

Continuous determination of volatile products in anaerobic fermenters by on-line capillary gas chromatography.

Bio-ethanol and biogas produced during the anaerobic conversion of organic compounds has been a subject of great interest since the oil crisis of the 1970s. In ethanol fermentation and anaerobic treatment of wastewaters, end-product (ethanol) and intermediate-products (short-chain fatty acids, SCFA) cause inhibition that results in reduced process efficiency. Control of these constituents is of utmost importance for bioreactor optimization and process stability. Ethanol and SCFA can be detected with precision by capillary gas chromatography usually conducted in off-line measurements. In this work, an on-line monitoring and controlling system was developed and connected to the fermenter via an auto-sampling equipment, which could perform the feeding, filtration and dilution of the sample and final injection into the gas chromatograph through an automation-based programmed procedure. The sample was continuously pumped from the recycle stream of the bioreactor and treated using a microfiltration unit. The concentrate was returned to the reactor while the permeate was quantitatively mixed with an internal standard solution. The system comprised of a gas chromatograph with the flow cell and one-shot sampler and a PC with the appropriate software. The on-line measurement of ethanol and SCFA, directly from the liquid phase of an ethanol fermenter and a high-rate continuous mode anaerobic digester, was accomplished by gas chromatography. Also, this monitoring and controlling system was proved to be effective in the continuous fermentation of alcohol-free beer.

On-line load monitoring of wastewaters with a respirographic microbial sensor.

This paper demonstrates the functionality, laboratory testing and field application of a microbial sensor that is capable of monitoring the organic pollution extent of wastewaters both off-line in a laboratory and on-line in a wastewater treatment plant. The biosensor was first developed in the laboratory using synthetic wastewater and then applied to monitor the effluent of the unit. The basic working principle of the biosensor is based on the on-line measurement of CO2 concentration in the off gas produced during carbon compound degradation by microbial respiration activities. CO2 concentration under operation conditions (constant oxygen flow rate, residence time and pH) is closely related to the extent of organic pollution (biochemical oxygen demand, chemical oxygen demand). CO2 monitoring is carried out by an infrared spectrometer, whereas current organic pollution is determined off-line according to the conventional 5-day lasting BOD analysis. Off gas analysis of CO2 concentration strongly correlates with off-line biochemical oxygen demand measurements allowing continuous on-line monitoring of the organic load within a wastewater treatment plant. Thus, real time process control and operation become feasible.

Continuous gluconic acid production by isolated yeast-like mould strains of Aureobasidium pullulans.

By extensive microbial screening, about 50 strains with the ability to secrete gluconic acid were isolated from wild flowers. The strains belong to the yeast-like mould Aureobasidium pullulans (de Bary) Arnaud. In shake flask experiments, gluconic acid concentrations between 23 and 140 g/l were produced within 2 days using a mineral medium. In batch experiments, various important fermentation parameters influencing gluconic acid production by A. pullulans isolate 70 (DSM 7085) were identified. Continuous production of gluconic acid with free-growing cells of the isolated yeast-like microorganisms was studied. About 260 g/l gluconic acid at total glucose conversion could be achieved using continuous stirred tank reactors in defined media with residence times (RT) of about 26 h. The highest space-time-yield of 19.3 g l(-1) x h(-1)) with a gluconic acid concentration of 207.5 g/l was achieved with a RT of 10.8 h. The possibility of gluconic acid production with biomass retention by immobilised cells on porous sinter glass is discussed. The new continuous gluconate fermentation process provides significant advantages over traditional discontinuous operation employing Aspergillus niger. The aim of this work was the development of a continuous fermentation process for the production of gluconic acid. Process control becomes easier, offering constant product quality and quantity.

Citric acid production by Candida strains under intracellular nitrogen limitation.

A suitable strain and important factors influencing citric acid formation in yeasts were identified. Candida oleophila ATCC 20177 was chosen as the best citric acid producer from several Candida strains. Yields of 50 g/l citric acid were produced in shake flask and 80 g/l in fed-batch fermentations with 1.5 and 3 g/l NH(4)Cl under non-optimized conditions. Ammonium nitrogen was identified as the limiting substrate for citrate formation. Citric acid excretion begins a few hours after exhaustion of nitrogen in the medium. The importance of intracellular nitrogen limitation was clarified by elemental analysis of C. oleophila biomass. The nitrogen content of C. oleophila biomass decreased from 7.45% during the growth phase to 3.96% in the production phase. The biomass contained less carbon and more trace elements in the growth phase compared with the production phase. Relatively high intracellular NH(4)(+) concentration of about 1.2 mg/g biomass (~37.4 mM) was found during the production phase. The low intracellular nitrogen content and increase of intracellular NH(4)(+) concentration, possibly caused by proteolysis following extracellular nitrogen exhaustion, trigger citric acid production. Intracellular nitrogen limitation and the increase in intracellular NH(4)(+) concentration are the most important factors influencing citric acid formation in yeasts.

Use of a microbial sensor: inhibition effect of azo-reactive dyes on activated sludge.

This paper summarizes the methodology utilized for measuring the toxic and inhibitory effects of azo-reactive dyes on the activity of activated sludge. The microbial sensor employed in this study consisted of a small-fluidized bed reactor in which the microbial mass was immobilized on spherical (diameter =1-2 mm) reticulated sinter glass carriers. To sustain a highly dense population of aerobic microbes, pure oxygen was supplied via a cylindrical chamber, which comprised part of the sample re-circulation system. The mean hydraulic retention time in the microbial sensor ranged between 30 and 40 min, while temperature was maintained at 30 degrees C and pH at 6.4. Inhibition of microbial activity (toxicity) was determined as the mean percent reduction in carbon dioxide production from microorganisms' respiration. Several azo-reactive dyes demonstrated toxicity when applied at a high concentration (2 g/l), however, a portion of the microbes showed tolerance to the dyes. Moreover, textile wastewater demonstrated very efficient biodegradation.