Hydrolysis of return sludge for production of easily biodegradable carbon: effect of pre-treatment, sludge age and temperature.

Return sludge from two Swedish and two Danish wastewater treatment plants were hydrolysed in laboratory reactors. Treatment plants with/without pre-sedimentation and with/without nitrification were represented. Soluble organic matter was produced from all types of sludge, but the yield was to a large extent dependent on what type of sludge was hydrolysed. Activated sludge from wastewater treatment plants without pre-treatment returned more soluble carbon after hydrolysis than sludge from treatment plants with pre-sedimentation. In addition, more soluble carbon was formed from non-nitrifying activated sludge than from nitrifying sludge. Moreover, the maximum yield of soluble COD at 10 degrees C was less than the yield at 20 degrees C. The initial hydrolysis rate was found to be between 0.35 and 1.8 mg soluble COD/(g VS x h). With the exception of one case, between 15 and 50% of the produced soluble COD was shown to be volatile fatty acids, a suitable carbon source for biological phosphorus removal. Nitrification rate measurements indicated that the viability of the activated sludge was not affected by the hydrolysis.     

Application of hydrothermal reaction for excess sludge reuse as carbon sources in biological phosphorus removal.

An application of hydrothermal reaction was investigated to reuse excess sludge as carbon sources for enhancement of biological phosphorus removal. Under the tested conditions, solubilization of treated excess sludge did not present much variation, sustaining around 65%, except the results obtained at 400 degrees C. Biodegradability of excess sludge was improved through its content change by the reaction, without much reduction of carbon contents even in 7 min. From the results of respirometric test, readily biodegradable substrate was found at 300 degrees C. Then its portion of reaction products increased with increasing reaction temperature. In the readily biodegradable substrate, acetic and propionic acid, which are useful carbon sources for phosphorus accumulating microorganism under anaerobic condition, increased with increasing reaction temperature. Hydrothermal reaction might be accepted as suitable pretreatment method to treat excess sludge prior to biological treatment process. This technology also secures excess sludge reuse, enhancing biological phosphorus removal and improvement of biological treatment process.     

Effect of enzymatic pretreatment on solubilization and volatile fatty acid production in fermentation of food waste.

Food waste can be a valuable carbon source in biological nutrient removal (BNR) systems because of the high C/N and C/P ratio. However, pretreatment is necessary to promote hydrolysis of food waste because of the high concentration of volatile solids associated with organic matter. The influence of the enzymatic pretreatment on acid fermentation of food waste was investigated in this study. Solubilization of particulate matter in food waste was carried out using commercial enzymes. The acidification efficiency and the volatile fatty acid (VFA) production potential of enzymatically pretreated food waste were examined. The highest volatile suspended solid (VSS) reduction was obtained with an enzyme mixture ratio of 1:2:1 for carbohydrase: protease: lipase. An optimum enzyme dosage for solubilization of food waste was 0.1% (V/V) with the enzyme mixture ratio of 1:2:1. In the acid fermentation of enzymatically pretreated food waste, the maximum VFA production and the highest VFA fraction in soluble COD (SCOD) were also achieved at 0.1% (V/N) of total enzyme dosage. Increase in VFA production at this level of enzyme dosage was over 300% compared with the control fermenter. The major form of VFA produced by fermentation was n-butyrate followed by acetate.     

Are standard wastewater treatment plant design methods suitable for any municipal wastewater?

The design and operational parameters of an activated sludge system were analyzed treating the municipal wastewaters in Istanbul. The design methods of ATV131, Metcalf & Eddy together with model simulations were compared with actual plant operational data. The activated sludge model parameters were determined using 3-month dynamic data for the biological nutrient removal plant. The ATV131 method yielded closer sludge production, total oxygen requirement and effluent nitrogen levels to the real plant after adopting correct influent chemical oxygen demand (COD) fractionation. The enhanced biological phosphorus removal (EBPR) could not easily be predicted with ATV131 method due to low volatile fatty acids (VFA) potential.     

Multi-component kinetics of activated sludge treatment of bleached kraft mill effluent.

This study investigated the discrepancies between the BOD removal rates measured during short term assays and those measured during continuous activated sludge treatment of bleached kraft mill effluent (BKME). A combination of batch tests and fed batch tests with oxygen uptake rate (OUR), chemical oxygen demand (COD), biochemical oxygen demand (BOD), and mixed liquor volatile suspended solids (MLVSS) measurements were used to characterize the degradation rates for the activated sludge treatment of BKME and to divide the soluble readily biodegradable substrate into two to five separate fractions based on biodegradation rates. The removal rates varied by over an order of magnitude between the most readily degradable substrates (1 x 10(-3) mg COD/mg MLVSS minute), and the more slowly degradable substrates (2 x 10(-5) mg COD/mg MLVSS minute). If the readily biodegradable fraction of BKME was modeled as one substrate, initial rate kinetic measurements from batch tests were heavily influenced by the fractions with the greatest degradation rates, while any remaining BOD in the treated effluent was predominantly from the slowly degradable fraction, giving inconsistent results. Taking the multi-component nature of the wastewater into account, batch test results can be used to predict fed-batch and continuous activated sludge reactor performance.     

Effects of solids concentration, pH and carbon addition on the production rate and composition of volatile fatty acids in prefermenters using primary sewage sludge.

Increasing evidence is emerging that the performance of enhanced biological phosphorus removal (EBPR) systems relies on not only the total amount but also the composition of volatile fatty acids (VFAs). Domestic wastewater often contains limited amounts of VFAs with acetic acid typically being the dominating species. Consequently, prefermenters are often employed to generate additional VFAs to meet the demand for carbon by EBPR and/or denitrification processes. Limited knowledge is currently available on the effects of operational conditions on the production rate and composition of VFAs in prefermenters. In this study, a series of controlled batch experiments were conducted with sludge from a full-scale prefermenter to determine the impact of solids concentration, pH and addition of molasses on prefermentation processes. It was found that an increase in solids concentration enhanced total VFA production with an increased propionic acid fraction. The optimal pH for prefermentation was in the range of 6-7 with significant productivity loss when pH was below 5.5. Molasses addition significantly increased the production of VFAs particularly the propionic acid. However, the fermentation rate was likely limited by the biological activity of the sludge rather than by the amount of molasses added.     

Application of excess activated sludge ozonation in an SBR Plant. Effects on substrate fractioning and solids production

This paper provides new insights on the application of the ozonation process for the reduction of the activated sludge production in a sequencing batch reactor (SBR). The study was performed in two identical lab-scale SBRs plant, one for experimental activities (Exp SBR) and one used as control (Control SBR), both fed with domestic sewage. A fraction of the activated sludge collected from the Exp SBR at the end of the aerobic react phase was periodically subjected to ozonation for 30 minutes at three different specific dosages (0.05, 0.07 and 0.37 g O(3)/gSS) and then recirculated before the beginning of the following cycle.Recirculation of the ozonated sludge to the Exp SBR did not appreciably affect the efficiency of the biological nitrogen and carbon removal processes. Nonetheless, an improvement of the denitrification kinetic was observed. Mixed liquor volatile and suspended solids (MLSS and MLVSS, respectively) concentrations in the reactor decreased significantly with time for long term application of the ozonation treatment. Kinetic batch tests on unstressed sludge taken from Control SBR indicated that the different oxidant dosages (0.05, 0.07 and 0.37 g O(3)/gSS) and durations of the ozonation process (10, 20 and 30 minutes) used remarkably affected chemical oxygen demand (COD) and organic nitrogen fractioning. In particular, soluble and biodegradable fractions seemed to be higher at lower dosage and longer contact time.     

Hydrolysis and fermentation of activated sludge to enhance biological phosphorus removal.

The conventional mainstream enhanced biological phosphorus removal (EBPR) process depends on the quality of the raw incoming wastewater. An alternative sidestream EBPR process is presented, where the substrates for storage by the polyphosphate accumulating organisms (PAOs) instead come from hydrolysis of the return activated sludge. This process is studied in full-scale at two treatment plants and quantified by means of phosphorus release rates and readily biodegradable COD (RBCOD) accumulation rates. It was seen that not only was a significant amount of RBCOD stored by PAOs but an approximately equal amount was accumulated in the sidestream hydrolysis tank and made available for the subsequent nitrogen removal process. The phosphorus release of the sludge with and without addition of different substrates was furthermore studied in laboratory scale. The study showed that the process is promising and in a number of cases will have significant advantages compared with the conventional mainstream EBPR     

Alternate anoxic/aerobic operation for nitrogen removal in a membrane bioreactor for municipal wastewater treatment

A large pilot-scale membrane bioreactor (MBR) with a conventional denitrification/nitrification scheme for municipal wastewater treatment has been run for one year under two different aeration strategies in the oxidation/nitrification compartment. During the first five months air supply was provided according to the dissolved-oxygen set-point and the system run as a conventional predenitrification MBR; then, an intermittent aeration strategy based on effluent ammonia nitrogen was adopted in the aerobic compartment in order to assess the impact on process performances in terms of N and P removal, energy consumption and sludge reduction. The experimental inferences show a significant improvement of the effluent quality as COD and total nitrogen, both due to a better utilization of the denitrification potential which is a function of the available electron donor (biodegradable COD) and electron acceptor (nitric nitrogen); particularly, nitrogen removal increased from 67% to 75%. At the same time, a more effective biological phosphorus removal was observed as a consequence of better selection of denitrifying phosphorus accumulating organisms (dPAO). The longer duration of anoxic phases also reflected in a lower excess sludge production (12% decrease) compared with the standard pre-denitrification operation and in a decrease of energy consumption for oxygen supply (about 50%).     

Simultaneous removal of organic and strong nitrogen from sewage in a pilot-scale BNR process supplemented with food waste.

As the sewerage system is incomplete, sewage in Korea lacks easily biodegradable organics for nutrient removal. In this country, about 11,400 tons of food waste of high organic materials is produced daily. Therefore, the potential of food waste as an external carbon source was examined in a pilot-scale BNR (biological nutrient removal) process for a half year. It was found that as the supply of the external carbon increased, the average removal efficiencies of T-N (total nitrogen) and T-P (total phosphorus) increased from 53% and 55% to 97% and 93%, respectively. VFAs (volatile fatty acids) concentration of the external carbon source strongly affected denitrification efficiency and EBPR (enhanced biological phosphorus removal) activity. Biological phosphorus removal was increased to 93% when T-N removal efficiency increased from 78% to 97%. In this study, several kinds of PHAs (poly-hydroxyalkanoates) in cells were observed. The observed PHAs was composed of 37% 3HB (poly-3-hydroxybutyrate), 47% 3HV (poly-3-hydroxyvalerate), 9% 3HH (poly-3-hydroxyhexanoate), 5% 3HO (poly-3-hydroxyoctanoate), and 2% 3HD (poly-3-hydroxydecanoate).     

Experience from 10 years of full-scale operation with enhanced biological phosphorus removal at Oresundsverket.

Ten years of full-scale experience with enhanced biological phosphorus removal (EBPR) has been evaluated. During the start-up period lack of carbon source was the main operational problem and a higher level of volatile fatty acids was secured by introducing a primary sludge hydrolysis. Acidic thermal sludge hydrolysis was used as the sludge treatment method at the plant during about three years. One effluent stream, rich in carbon and precipitant, was brought back to the process leading to an improvement of the phosphorus removal both by an improved biological process and chemical precipitation. A quite stable process of EBPR was developed with low levels of effluent phosphorus concentration. Stringent effluent discharge limits during short evaluation periods necessitated a continued work for improvement of the short-term stability. During periods with lack of carbon, such as industrial holiday or rainy periods, both simultaneous precipitation and reduced aeration have been successfully tested as strategies for securing low levels of effluent phosphorus.     

Aerobic biological treatment of thermophilically digested sludge

Aerobic biological treatment of digested sludge was studied in a continuously operated laboratory set-up. An aerated reactor was filled with thermophilically digested sludge from the Moscow wastewater treatment plant and inoculated with special activated sludge. It was then operated at the chemostat mode at different flow rates. Processes of nitrification and denitrification, as well as dephosphatation, occurred simultaneously during biological aerobic treatment of thermophilically digested sludge. Under optimal conditions, organic matter degradation was 9.6%, the concentrations of ammonium nitrogen and phosphate decreased by 89 and 83%, respectively, while COD decreased by 12%. Dewaterability of digested sludge improved significantly. The processes were found to depend on hydraulic retention time, oxygen regime, and temperature. The optimal conditions were as follows: hydraulic retention time 3-4 days, temperature 30-35 degrees C, dissolved oxygen levels 0.2-0.5 mg/L at continuous aeration or 0.7-1 mg/L at intermittent aeration. Based on these findings, we propose a new combined technology of wastewater sludge treatment. The technology combines two stages: anaerobic digestion followed by aerobic biological treatment of digested sludge. The proposed technology makes it possible to degrade the sludge with conversion of approximately 45% volatile suspended solids to biogas, to improve nitrogen and phosphorus removal in reject water from sludge treatment units, and to achieve removal of malodorous substances after 8-9 days of anaerobic-aerobic sludge treatment.     

Re-use of winery wastewaters for biological nutrient removal.

The aim of this study was to evaluate the feasibility of the re-use of the winery wastewater to enhance the biological nutrient removal (BNR) process. In batch experiments it was observed that the addition of winery wastewater mainly enhanced the nitrogen removal process because of the high denitrification potential (DNP), of about 130 mg N/g COD, of the contained substrates. This value is very similar to that obtained by using pure organic substrates such as acetate. The addition of winery wastewater did not significantly affect either phosphorus or COD removal processes. Based on the experimental results obtained, the optimum dosage to remove each mg of N-NO3 was determined, being a value of 6.7 mg COD/mg N-NO3. Because of the good properties of the winery wastewater to enhance the nitrogen removal, the viability of its continuous addition in an activated sludge pilot-scale plant for BNR was studied. Dosing the winery wastewater to the pilot plant a significant increase in the nitrogen removal was detected, from 58 to 75%. The COD removal was slightly increased, from 89 to 95%, and the phosphorus removal remained constant.     

Nitrogen removal in an upflow sludge blanket (USB) reactor combined by aerobic biofiltration systems.

A new nitrogen removal process (up-flow sludge blanket and aerobic filter, USB-AF) was proposed and tested with real sewage. In the USB reactor, the larger part of influent organic and nitrogen matters were removed, and ammonia was effectively oxidized in the subsequent aerobic filter. The role of the aerobic filter was to convert ammonia into nitrate, an electron acceptor that could convert soluble organic matters into volatile suspended solid (VSS) in the USB. The accumulated as well as influent VSS in the USB was finally degraded to fermented products that were another good carbon source for denitrification. Total COD, settleable COD and soluble COD in the raw sewage were 325, 80 and 140 mg/l, respectively. Most unsettleable COD as well as some SCOD in the influent was successfully removed in the USB. TCOD removal in the anoxic filter was by denitrification with the recycled nitrate. Low COD input to the aerobic filter could increase nitrification efficiency, reduce the start-up period and save the aeration energy in the USB-AF system. About 95% of ammonia was nitrified in the aerobic filter with no relation to the influent ammonia concentration. Denitrification efficiency of the recycled nitrate in the anoxic filter was about 85, 83, and 72% at recycle ratios of 100, 200, and 300%, respectively. T-N removal efficiency was 70% at recycle ratio of 300%.     

Integrated chemical, physical and biological processes modelling of anaerobic digestion of sewage sludge.

The biological kinetic processes for anaerobic digestion (AD) are integrated into a two phase subset of a three phase mixed weak acid/base chemistry kinetic model. The approach of characterising sewage sludge into carbohydrates, lipids and proteins, as is done in the International Water Association (IWA) AD model No 1 (ADM1), requires measurements that are not routinely available on sewage sludges. Instead, the sewage sludge is characterised with the COD, carbon, hydrogen, oxygen and nitrogen (CHON) composition and is formulated in mole units, based on conservation of C, N, O, H and COD. The model is calibrated and validated with data from laboratory mesophilic anaerobic digesters operating from 7 to 20 d sludge age and fed a sewage primary and humus sludge mixture. These digesters yielded COD mass balances between 107-109% and N mass balances between 91-99%, and hence the experimental data is accepted as reasonable. The sewage sludge COD is found to be 32-36% unbiodegradable (depending on the kinetic formulation selected for the hydrolysis process) and to have a C3.5H7O2N0.196 composition. For the selected hydrolysis kinetics of surface mediated reaction (Contois), with a single set of kinetic and stoichiometric constants, for all retention times good correlation is obtained between predicted and measured results for: (i) COD; (ii) free and saline ammonia (FSA); (iii) short chain fatty acids (SCFA); (iv) H2CO3 * alkalinity; (v) pH of the effluent stream; (vi) CO2; and (vii) CH4 gases in the gas stream. The measured composition of primary sludge from two local wastewater treatment plants ranged between C3.38H7O1.91 N0.21 and C3.91H7O2.04N0.16. The predicted composition based on mass balances is therefore within 5% of the average measured composition providing persuasive validation of the model.     

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.     

Anaerobic treatment of a medium strength industrial wastewater at low-temperature and short hydraulic retention time: a pilot-scale experience

The aim of this work was to evaluate the performance of a pilot-scale upflow anaerobic sludge blanket (UASB) reactor during the treatment of cereal-processing industry wastewater under low-temperature conditions (17 degrees C) for more than 300 days. The applied organic loading rate (OLR(appl)) was gradually increased from 4 to 6 and 8 kg COD(sol)/m3d by increasing the influent soluble chemical oxygen demand (COD(sol)), while keeping the hydraulic retention time constant (5.2 h). The removal efficiency was high (82 to 92%) and slightly decreased after increasing the influent COD(sol) and the OLR(appl). The highest removed organic loading rate (OLR(rem)) was reached when the UASB reactor was operated at 8 kg COD(sol)/m3d and it was two times higher than that obtained for an OLR(appl) of 4 kg COD(sol)/m3d. Some disturbances were observed during the experimentation. The formation of biogas pockets in the sludge bed significantly complicated the biogas production quantification, but did not affect the reactor performance. The volatile fatty acids in the effluent were low, but increased as the OLR(appl) increased, which caused an increment of the effluent COD(sol). Anaerobic treatment at low temperature was a good option for the biological pre-treatment of cereal processing industry wastewater.     

Application of a membrane bioreactor for the treatment of low loaded domestic wastewater for water re-use.

This paper describes the demonstrative scale application of a membrane biological reactor (MBR) for low loaded domestic wastewater with low attitude to biological treatment (carbon/nitrogen approximately 5). The biological process was managed by the automatically controlled alternate cycles allowing for re-use purposes with a remarkable reduction of the operational costs. The global process evaluation revealed the system capability of obtaining high nitrogen removal (effective nitrogen removal of 69%) thanks to its high flexibility related to the hourly loading fluctuation. Moreover, high removal of heavy metals and polycyclic aromatic hydrocarbons (PAH) was obtained due to the perfect retention capability of the membranes. In-depth studies were conducted to determine the process behaviour for activated sludge over aeration and with addition of exogenous carbon. Limitation of sludge over aeration and energy savings were observed with a gradient air supplying method. The addition of exogenous carbon (acetic acid up to carbon/nitrogen approximately 9) led to complete nitrogen removal (Ed = 96%) and permitted biological phosphorus uptake. In conclusion, it was been found that the coupled process alternated cycles-MBR had the capacity to remove COD, BOD, N, P and suspended solids, as well as heavy metals and organic micropollutants, resulting in high quality effluent suitable for re-use purposes.     

Effect of pretreatment on acid fermentation of organic solid waste.

The effect of various pretreatment methods on acidogenesis of food waste from a cafeteria was investigated. Thermal, enzymatic, and combined thermal-enzymatic batch pretreatment were conducted. Solubilization of food waste in thermal or enzymatic pretreatment increased with either increasing thermal duration or enzyme dosage, respectively. An optimal condition in thermal or enzymatic pretreatment was 60 min of thermal processing or 0.1% (v/v) of enzyme dosage level, respectively, based on volatile suspended solids (VSS) reduction. In the combined pretreatment, increased thermal duration also increased VSS reduction and the solubilization efficiency was higher than that in only enzymatic or thermal pretreatment. The maximum volatile fatty acids (VFAs) production and the highest VFAs fraction in soluble COD in the acid fermentation were achieved with the food waste after 60 min of thermal processing followed by 0.1% (v/v) enzymatic treatment. Increase in VFAs production of the fermenter was over 380% compared to the control fermenter without any pretreatment.     

Utilising laboratory experiments as a first step to introduce primary sludge hydrolysis in full-scale.

Laboratory experiments have been utilised as a tool to determine the possible yield of soluble COD and VFAs from settled influent and preprecipitated sludge at Klagshamn wastewater treatment plant and to determine the degradability of the organic matter. The release of ammonium and orthophosphate from the hydrolysed sludge has been estimated. It is possible to produce soluble organic matter of good quality from settled influent and preprecipitated sludge at Klagshamn WWTP. Denitrification rates of 3.1 mg NO3-N/(g VSS x h) were found for the hydrolysate in laboratory tests. Owing to the low phosphate concentrations in the hydrolysate from preprecipitated sludge, it shows a potential for postdenitrification despite stringent phosphorus outlet demands. Calculations based on data gained from the laboratory experiments show that about 50% of the external carbon source used today can be saved with minor changes in the plant operation except that the hydrolysis has to be established. Based on these findings, full-scale experiments with internal hydrolysis in the primary clarifiers were scheduled in one of the two lines at Klagshamn WWTP for the summer of 2007.