Showing papers on "Hydraulic retention time published in 1998"

Production of Soluble Microbial Products (SMP) in an Anaerobic Baffled Reactor: Composition, Biodegradability, and the Effect of Process Parameters

Abstract: The composition and biodegradability of effluents from biological treatment plants is of increasing interest due to tightening effluent standards. One of the main constituents of effluents are soluble microbial products (SMPs) produced in the reactor during metabolism and endogenous decay. The objective of this preliminary work was to examine the influence of operating parameters such as temperature, hydraulic retention time (HRT), and organic loading rate (OLR) on the production of SMPs in a compartmentalised anaerobic baffled reactor (ABR) fed a simple sucrose-nutrients substrate. In addition, in order to gain greater insight into the SMPs produced, the composition and anaerobic biodegradability of various molecular weight fractions were analysed. It was found that decreasing operating temperature resulted in slight increases in SMP production, a decreased rate of degradation, and hence increases in effluent SMPs. In addition, decreasing HRTs, and increasing OLRs, also resulted in increased effluent SMP...

Effects of Pre-Ozonation in Refractory Leachate Treatment by the Biological Activated Carbon Fluidized Bed Process

Abstract: The removal efficiency of refractory landfill leachate organics by the biological activated carbon fluidized bed (BACFB) process was compared with and without pre-ozonation. Furthermore, the effects of ozonation upon the characteristics of the refractory leachate such as UV absorbance, dissolved organic carbon (DOC), BOD5, adsorbability, molecular size distribution and the content of humic substances were evaluated. The effectiveness of pre-ozonation in treating a refractory landfill leachate through the BACFB process was confirmed. The steady-state DOC removal was enhanced from 42% to 57% at a hydraulic retention time of 24 h. This improvement was due mainly to the pre-ozonation-enhanced biodegradability of the high-molecular-weight organics that could not be removed by the BACFB process alone. Ozonation of the leachate resulted in an exponential decrease in UV absorbance and a linear decrease in DOC with increasing the contact time. High-molecular-weight leachate organics degraded through ozonation to o...

Influence of organic loading rate and hydraulic retention time on the efficiency of a UASB bioreactor treating a canning factory effluent

Abstract: A mesophylic laboratory-scale upflow anaerobic sludge bed bioreactor design was evaluated for the treatment of a carbohydraterich effluent from the canning industry. The bioreactor was inoculated with 500 g of anaerobic granules and after the system had stabilised the hydraulic retention time (HRT) was set at 24 h and the substrate pH poised at 8.0 to prevent the effect of rapid acidification. In the first experimental study the chemical oxygen demand (COD) was increased stepwise from 2300 to a full strength of 4000 mg.l -1 . In the second study the organic loading rate was increased by shortening the HRT (24 to 8 h) to give on organic loading rate increase from 3.95 to 10.95 kgCOD.m -3 d -1 with an average COD removal of 90 to 93% and removal rate of 9.8 kgCOD.m -3 .d -1 . However, the recovery rate of the system at HTR values below 10 h was found to be very slow suggesting that the system had reached its minimum HRT. This was confirmed by the stabilisation of the granule bed. An HRT of 10 h was thus taken as the optimum operational HRT. Since neutralisation costs would intluence economic aspects of the process, the influence of lower pH values was investigated in the third study where the pH of the canning effluent was lowered from 8.0 to 5.0. At the lower pH the COD removal dropped drastically, the biogas production decreased and the digester effluent pH dropped to 6.2. It was clear from the slow recovery of the digester and the low COD removal (66,1%) that the lower end of the operational pH had been reached and any further lowering of the substrate pH would lead to system failure. The economic implication of being able to operate at pH 5.5 means that fresh canning effluent can be introduced into the digester without any neutralisation, is considerable.

Removal of organic pollutants and of nitrate from wastewater from the dairy industry by denitrification

Abstract: The aim of this work was to remove nitrate-N and organic pollutants from wastewater of the dairy industry by denitrification. An artificially prepared wastewater, containing 250 mg/l nitrate-N and 1.5 g/l whey powder, was completely denitrified with removal of 90%–93% of the chemical oxygen demand (COD) of the whey powder by suspended or immobilized mixed cultures and by a suspended or immobilized pure culture that was isolated from the mixed culture inoculum. For the above COD/nitrate-N ratio of 6:1, the results indicated that the organic compounds of the wastewater served as electron donors for complete denitrification and that there was no need to add an external carbon source. In batch denitrification assays the suspended or immobilized mixed cultures proved to be more active and reacted faster than the isolated pure cultures. In continuous denitrification processes with immobilized pure or mixed cultures, the alginate beads, used for immobilization, were not stable for more than 12 days of incubation. The mixed free cultures removed the nitrate-N and COD continuously with no change of their activity for at least 15 days at an optimum hydraulic retention time of 0.27 days with a loading rate of 900 mg nitrate-N l−1 day−1.

Design of acidogenic reactors for the anaerobic treatment of the organic fraction of solid food waste

Abstract: Volatile Fatty Acids (VFA) production by anaerobic fermentation of organic solid wastes was studied at laboratory scale. The influence of initial substrate concentration was evaluated on VFA production. Completely mixed reactors (0.9 l) were used at mesophilic temperature (35 °C). Food wastes had 43.8% Total Solids content. Three dilutions of substrate (1/25, 1/10 and 1/5) corresponding to 1.75%, 4.38% and 8.76% of Total Solids and five values of Organic Loading Rates: 2, 5, 10, 12.5 and 25 kg COD/m3 d were studied. It was found that substrate 1/10 led to 14 g VFA/l at a loading rate of 12.5 kg COD/m3 d and an hydraulic retention time of 3.7 d. The main VFA produced were especially acetate and butyrate. Substrate diluted 1/5 led to 26.1 g VFA/l at a loading of 5 kg COD/m3 d and an hydraulic retention time of 15.1 d, but biomass production was not optimal. In a second study, a cascade of three reactors was used. An effluent with 42 g VFA/l was obtained at steady-state conditions at a loading of 12.5 kg of COD/m3 d and an hydraulic retention time of 12.5 d. The distribution of VFA was the following: 36% of propionate, 34% of acetate and 22.5% of butyrate.

The influence of start-up strategies on the performance of an anaerobic baffled reactor

Abstract: The start-up and performance of two 10 litre anaerobic baffled reactors (ABR), both of which contained eight compartments and were operated under mesophilic conditions (35°C), were investigated. Two start-up strategies were evaluated: one with a constant hydraulic retention time (HRT) and a stepwise doubling of the organic feed strength (ABR 1), the other with a constant organic strength (4 g l−1 chemical oxygen demand COD, as sucrose) coupled to a systematic halving of HRT (ABR 2). The final desired baseline conditions were a feed strength of 4 g COD−1 and 20 hours HRT. The best stability and COD removal was demonstrated by the reactor started at a long retention time (ABR 2). The biomass (volatile suspended solids, VSS) lost from each reactor was greater in ABR 1 than in ABR 2 (200 g compared to 100 g after 140 days), and it was hypothesised that in ABR 1 low retention time and channelling controlled biomass washout, whereas in ABR 2 washout appeared to be caused by gas mixing. Microbial floc sizes were...

Effect of Hydraulic Retention Time on Nitrification in an AirLift Biological Reactor

Abstract: The occurrence of nitrogenous compounds in industrial effluents at concentration levels above legal limits, is a well-known and serious pollution problem for the receiving body. The biological process for the removal of these substances, commonly referred to as ammoniacal nitrogen, is known as nitrification. Bacteria involved are mainly of the genuses Nitrosomonas and Nitrobacter. The aim of the present work was to study the effect of the hydraulic retention time (HRT) on the efficiency of ammonia removal from a petroleum refinery effluent using activated carbon particles as a biofilm support in an airlift bioreactor. The experiments were carried out using HRTs, equal to six, eight and ten hours. The results show that HRT equal to 8 and 10 hours were enough to reduce ammoniacal nitrogen concentration to levels below permited legal limits (5mg/L NH3-N). The reactor nitrifying performance was maximized at 85% removal of ammoniacal nitrogen, for a HRT equal to 10 hours.

Simultaneous organic and nutrient removal from municipal wastewater by bsacnr process

Abstract: Biological nutrient removal was investigated under a biological synthetic activated ceramic nutrient removal (BSACNR) process. Tests were made to establish whether organic compounds and nutrients (N, P) from municipal wastewater were eliminated effectively in a lab-scale BSACNR process by increasing the hydraulic retention time (HRT) from 4 hr to 10 hr. In the system, synthetic activated ceramic (SAC) media were packed in each reactor for attached growth of both nitrifying bacteria and denitrifying bacteria; the sludge of the clarifier was returned to the anaerobic reactor to release phosphate. In this configuration, nitrification, denitrification and phosphorus removal could be performed at their respective conditions. The influent was synthetic wastewater, and the mean concentration of COD, NH+ 4 -N and T-P in the influent was about 200 mg/L, 20 mg/L and 8 mg/L, respectively. At a total HRT of the system of 4-10 hr, the system worked successfully obtaining the removal of COD, NH+ 4-N, T-N and T-P: 90.5-97.5%, 72.9-94.4%, 56.5-73.7% and 36.0-61.1%, respectively. The results of this research show that a biological synthetic activated ceramic nutrient removal (BSACNR) process packed with SAC media could be applicable for treatment of organic and nutrient from municipal wastewater.

Treatment of cyanide-containing wastewater from the food industry in a laboratory-scale fixed-bed methanogenic reactor

Abstract: During the process of producing cassava starch from Manihot esculenta roots, large amounts of cyanoglycosides were released, which rapidly decayed to CN− following enzymatic hydrolysis. Depending on the varying cyanoglycoside content of the cassava varieties, the cyanide concentration in the wastewater was as high as 200 mg/l. To simulate anaerobic stabilization, a wastewater with a chemical oxygen demand (COD) of about 20 g/l was prepared from cassava roots and was fermented in a fixed-bed methanogenic reactor. The start-up phase for a 99% degradation of low concentrations of cyanide (10 mg/l) required about 6 months. After establishment of the biofilm, a cyanide concentration of up to 150 mg CN−/l in the fresh wastewater was degraded during anaerobic treatment at a hydraulic retention time of 3 days. All nitrogen from the degraded cyanide was converted to organic nitrogen by the biomass of the effluent. The cyanide-degrading biocoenosis of the anaerobic reactor could tolerate shock concentrations of cyanide up to 240 mg CN−/l for a short time. Up to 5 mmol/l NH4Cl (i.e. 70 mg N/l = 265 mg NH4Cl/l) in the fresh wastewater did not affect cyanide degradation. The bleaching agent sulphite, however, had a negative effect on COD and cyanide removal. For anaerobic treatment, the maximum COD space loading was 12 g l−1 day−1, equivalent to a hydraulic retention time of 1.8 days. The COD removal efficiency was around 90%. The maximum permanent cyanide space loading was 50 mg CN− l−1 day−1, with tolerable shock loadings up to 75 mg CN− l−1 day−1. Under steady-state conditions, the cyanide concentration of the effluent was lower than 0.5 mg/l.

Characterization of and bioproduction of short-chain organic acids from mixed dairy-processing wastewater

Abstract: Mixed wastewater from the cheese-processing industry was characterized and biologically treated to produce short-chain organic acids (i.e., acetic, propionic, butyric, and valeric acids) in laboratory-scale continuously stirred tank reactors. A total of 97.7% of total chemical oxygen demand (TCOD) in this wastewater was due to the presence of lactose, lactate, protein, and fat, which respectively composed 66.6%, 10.6%, 17.2%, and 3.3% of the TCOD. A constant inoculum system, an anaerobic continuously stirred tank reactor (CSTR) with 1.5 L working volume, was used to minimize the experimental error due to the use of inconsistent inoculum. The inoculum system was operated with dilute mixed waste, 5000 mg soluble COD/L at pH 6.5 and 35°C at 0.5 days hydraulic retention time (HRT). Two identical 2.8 L anaerobic CSTRs equipped with temperature and pH controllers were separately used to evaluate system performance. Several different HRTs (0.6 days, washout points) were randomly tried and the organics in the wastewater were converted to these four organic acids at up to 62.4% at pH 7.01, 36.2°C, and 0.45 days HRT.

Membrane bioreactor for drinking water denitrification

Abstract: The aim of this study is to evaluate the performance of a membrane bioreactor with cell recycle to be used for drinking water denitrification, when operated with a high nitrate load (up to 7.68 kgNO3−/m3 day) and low hydraulic retention time (down to 0.625 h). Nitrate and nitrite were always completely removed for all the operational conditions used. The effluent's nitrite concentration kept below 0.1 mg NO2−/l with exception of a short period, during the reactor start-up, when it accumulates. The performance of the membrane bioreactor was also evaluated using a groundwater containing 148 mg NO3−/l. Nitrate and nitrite concentration in the effluent were below the recommended values for drinking water when the reactor was controlled at pH 7.0. The membrane flux decreases during operation as a consequence of membrane fouling. The flux decrease was more severe during operation with synthetic medium than with contaminated groundwater due to the existence of molecular complexes in the synthetic broth. A backshock technique was used to reduce the surface fouling of the membrane. Combining this technique with the use of a reserve asymmetric structured membrane it was found that the membrane flux remains nearly unchanged.