Longer Hydraulic Retention Time Research Articles

Effect of HRT and MLSS on THM precursor removal in the activated sludge process

This study was conducted to evaluate the effect of hydraulic retention time (HRT) and mixed liquor suspended solids (MLSS) on trihalomethane (THM) precursor removal in the activated sludge process and to determine the effect of chemical properties of the organic components on trihalomethane formation potential (THMFP). A laboratory-scale reactor for the activated sludge process was used with synthetic domestic wastewater as influent. Experimental conditions, HRT and MLSS, were varied and the effluents were evaluated in terms of DOC and THMFP. Hydrophilicity, hydrophobicity and chemical functional groups were analyzed to determine the chemical parameters that may affect THMFP. Optimum removal of THMFP was achieved at HRT 24 h and at MLSS of ca. 2500 mg l −1. Due to the organic metabolites produced in the activated sludge floc, better DOC or THMFP removal was not observed at longer HRTs and larger MLSS. Decrease in DOC did not result in comparable removal of THMFP. This was attributed to two factors: the higher reactivity of the hydrophobic organic carbon (18.5 μg THMFP mg −1 C) compared to the hydrophilic organic carbon (6.2 μg THMFP mg −1 C) and the nonremoval of the remaining hydrophobic THM precursors in HRTs longer than 8 h. The activated sludge process preferentially removed the hydrophilic organic substances which was shown to have less potential to form THM. The activated sludge process effluents exhibited similar chemical characteristics compared with various types of treated industrial wastewaters determined in a previous study. The activated sludge process effluents contained larger amounts of the hydrophilic organic substances compared to the hydrophobic fractions. Larger amounts of the carboxylic and phenolic-OH functional groups were determined in the hydrophilic fractions than in the hydrophobic fractions. The equation obtained for THMFP prediction based on bulk parameters and specific functional groups of the treated industrial wastewaters was also found to be applicable on the activated sludge process effluents. The equation suggests that bulk THMFP was influenced by UV 260 and organic nitrogen of both the hydrophilic and the hydrophobic fractions, and phenolic-OH of the hydrophilic fractions.

Effect of sulfate concentration and scraping on aerobic fixed biofilm reactors

The influence of the sulfate concentration and biofilm scraping on the treatment performance and biofilm structure of aerobic fixed film reactors was studied in rotating tubular reactors (RTRs). The RTRs operated at 22(±3)°C treating diluted milk powder (0.9 g COD/1; COD/SO 2− 4 ratio of 20) at an organic loading rate of 30 g COD per m 2 carrier material per day. The reference RTR gave the following removal efficiencies: COD tot, 76%; COD sol, 91%; N totN, 51%; N orgN, 70%; PO 3− 4P, 45%. COD removal was linear as a function of the reactor length (2 mg COD/cm reactor). The biomass yield coefficient amounted to 0.44(±0.13) g DM/g COD removed. High sulfate concentrations appreciably reduced the yield to 0.28(±0.06) g DM/g COD rem but did not significantly influence the COD and nutrient removal capacity, despite the longer hydraulic retention time (12′07″) compared to the reference RTR (6′25″). Daily removal of about 15% of the biofilm by biomass scraping did not significantly affect the reactor performance. Biofilm samples growing in the reference and the sulfate amended reactor showed comparable specific oxygen uptake rates (15 mg O 2/g VS · h). In the reactor treating sulfate-rich wastewater, large populations (6.3–7.5 log CFU/g VS) of lactate, acetate, propionate and butyrate oxidizing sulfate reducing bacteria (SRB) were present. Their number as well as their sulfate reducing activity (13.5 ± 9.5 mg SO 2− 4S/g VS · h) was higher compared to the reference RTR. The addition of molybdate (30 mM) to the influent showed that SRB could account for 50% and 10% of the COD removal in the sulfate amended and reference RTR, respectively. It is concluded that wastewaters with a low COD SO 2− 4 ratio are particularly suited for aerobic fixed film processes because of the low waste biomass production.

Formation and growth of heterotrophic aerobic biofilms on small suspended particles in airlift reactors

In this article, the conditions for aerobic biofilm formation on suspended particles, the dynamics of biofilm formation, and the biomass production during the start-up of a Biofilm Airlift Suspension reactor (BAS reactor) have been studied. The dynamics of biofilm formation during start up in the biofilm airlift suspension reactor follows three consecutive stages: bare carrier, microcolonies or patchy biofilms on the carrier, and biofilms completely covering the carrier. The effect of hydraulic retention time and of substrate loading rate on the formation of biofilms were investigated. To obtain in a BAS reactor a high biomass concentration and predominantly continuous biofilms, which completely surround the carrier, the hydraulic retention time must be shorter than the inverse of the maximum growth rate of the suspended bacteria. At longer hydraulic retention times, a low amount of attached biomass can be present on the carrier material as patchy biofilms. During the start-up at short hydraulic retention times the bare carrier concentration decreases, the amount of biomass per biofilm particle remains constant, and biomass increase in the reactor is due to increasing numbers of biofilm particles. The substrate surface loading rate has effect only on the amount of biomass on the biofilm particle. A higher surface load leads to a thicker biofilm.A strong nonlinear increase of the concentration of attached biomass in time was observed. This can be explained by a decreased abrasion of the biofilm particles due to the decreasing concentration of bare carriers. The detachment rate per biofilm area during the start-up is independent of the substrate loading rate, but depends strongly upon the bare carrier concentration.The Pirt-maintenance concept is applicable to BAS reactors. Surplus biomass production is diminished at high biomass concentrations. The average maximal yield of biomass on substrate during the experiments presented in this article was 0.44 +/- 0.08 C-mol/C-mol, the maintenance value 0.019 +/- 0.012 C-mol/(C-mol h). The lowest actual biomass yield measured in this study was 0.15 C-mol/C-mol.

Performance of upflow anaerobic biofilter process in pharmaceutical wastewater treatment

Anaerobic processes have become increasingly important in the treatment of complex industrial wastewaters, which may not only have high concentrations of organics but may contain toxic, persistent/recalcitrant substances. The present paper reports an investigation into the start-up characteristics, feasibility and performance of the Upflow Anaerobic Biofilter Process (UABP) for treating a pharmaceutical effluent, which would be typical of such wastewaters, with an organic concentration of 20,000 mg L −1 COD and two branched-chain fatty acids believed to be persistent/recalcitrant, pivalic acid (PA) and 2-ethylhexanoic acid (2-EHA). In addition to these, there is a readily degradable component, acetic acid. The data from the start-up process beginning with biomass seeding to stable operation showed that the UABP could be satisfactorily started up in two months by using a multiple seeding protocol. Hydraulic Retention Time (HRT) was found to be an important key parameter which can improve the removal rates of all targeted substances. Tracking results derived over a year indicated that the process was able to achieve stable performance at 15 days HRT. Increasing the HRT to 20 days substantially improved removal rates of COD and 2-EHA to 93% and 100%, respectively. This longer HRT particularly improved the removal of pivalic acid from 15% to 90%. The biofilter was then able to maintain this good performance when HRT was subsequently decreased in steps to 1.2 days HRT which brought it towards failure. This investigation demonstrated that the UABP could treat pharmaceutical wastewater containing persistent organics effectively.

Effect of HRT on Acidogenic Digestion of Primary Sludge

A series of bench‐scale, continuous‐flow experiments, using primary sludge from a local municipal wastewater‐treatment plant, has been employed to investigate the effect of hydraulic retention time (HRT) on the acid‐phase anaerobic digestion process. Results show that both volatile fatty acid (VFA) production and chemical oxygen demand (COD) solubilization increase significantly with increasing HRT up to 12 h, but drop moderately at a longer HRT. Acetic acid and propionic acid are the main VFAs formed, averaging 46% and 32% of the total, respectively. Variation in HRT has a profound effect on organic substrate degradation as well. Regardless of the prevailing experimental conditions, lipids and carbohydrates are converted at higher percentages than proteins. Furthermore, protein utilization appears to be independent of reactor configuration, but carbohydrate and lipid degradation patterns are a function of the reactor regime.

Biological Removal of Chlorate from Bleaching Plant Effluent

Biological chlorate removal was studied on a laboratory and a pilot scale with the aim of optimizing process design and operating conditions with respect to process efficiency, stability and economy. The results showed a suspended-carrier biofilm process design to be suitable for biological chlorate removal. In the laboratory tests, at pH 7 and 37°C, a complete removal of chlorate could be maintained at hydraulic retention times (HRTs) as short as 24 min. A longer HRT (1.5 h) was required for complete chlorate removal in the pilot test, due to a lower degree of filling with carrier material (25% versus 50% of the reactor volume), higher process temperature, and leakage of oxygen into the process. However, it is assumed that the loading capacity of a large-scale process would approach that of the laboratory system if the operating conditions were the same. Laboratory tests showed chlorate reduction to be possible within a wide range of pH values and temperatures, although the process stability and loading capacity were strongly affected by changes in these parameters. The results of the laboratory and pilot scale studies, using a suspended-carrier process design, show biological treatment to be an economically viable and efficient process for the removal of chlorate from bleaching plant effluents.

Biological Treatment of a High Salinity Chemical Industrial Wastewater

Various laboratory-scale process configurations were tested for the biological treatment of a combined wastewater stream of several chemical factories. The untreated wastewaters, rich in halogenated organics (1250±389 mg/l DOC), were also highly saline (32±11 g/liter TDS 550°C) and toxic (Microtox™ EC50 = 1.5±2.0%). Biphasic (anaerobic/aerobic) laboratory bench-scale reactor systems yielded reduction of dissolved organic carbon by 70 to 84%, in the absence and presence of powdered activated carbon, respectively. The anaerobic phase proved to be essential in all systems, both for dissolved organic carbon removal and for detoxification. Similar efficiencies were obtained in either activated sludge or aerated lagoon type reactors, but in the latter case, longer hydraulic retention times were required. DOC removal was found to decrease with increased salt concentration; however, a 50% efficiency was achieved even at 90 g/l TDS. Toxicity elimination as judged by the Microtox™ assay was highly variable in the absence of activated carbon but stable and efficient in its presence.

The effect of hydraulic retention time on the hybrid anaerobic baffled reactor performance at constant loading

An experimental study was undertaken to determine the effect of hydraulic retention time (HRT) on a 1501 hybrid anaerobic baffled reactor (HABR) at constant organic loading 10 kg m −3 d −1. When the HRT was gradually changed from 1 day to 6 days, i.e. the feed concentration changed from 10 g 1 −1 to 60 g 1 −1 the COD reduction dropped from 75% to about 40%, the gas production rate decreased from 5.2 m 3 m 3 d −1 to 3.4 m 3 m 3 d −1, and the effluent volatile fatty acids (VFA) concentration increased from 430 mg 1 −1 to about 16,000 mg l −1 Thus methanogenic inhibition occurred at longer HRTs. The experimental results strongly suggested that the HRT is a significant factor for HABR performance.

Mesophilic methane fermentation of low-solids dairy-cattle waste

Methane fermentation of low-solids dairy waste was studied in horizontal, unagitated, isothermally maintained, tubular fermenters. The methane fermentation characteristics at temperatures of 20, 30 and 40°C and hydraulic retention times (HRTs) of 1, 2, 4, 6, 8 and 10 days are reported. Methane production per fermenter volume was greatest at a 1 day HRT; production of 0·22, 0·31 and 0·32 m 3 of methane/m 3 fermenter volume/day occurred at 20, 30 and 40°C, respectively. Maximum methane production per unit of substrate fed, however, occurred at longer HRTs. Maximum methane production was: 0·55 m 3 at 20°C and an 8 day HRT; 0·55 m 3 at 30°C and an 8 day HRT; 0·82 m 3 at 40°C and an 8 day HRT (all per kg volatile solids fed).

Limestone Bed Anaerobic Filter for Swine Manure — Laboratory Study

ABSTRACT PLEXIGLASS tubes (8.7 cm LD. x 65 cm long) and filled with 2 cm diameter limestock rock were used as lab scale upflow anaerobic filters to test screened (500 micron mesh) swine manure. Total volume of the filters was 3.3 L with a 2.0 L hydraulic void volume. The filters were operated at hydraulic retention times (HRT) of 1, 3, 6 and 9 days and organic loadings of 0.5, LO, 2.0, 4.0 and 6.0 kg VS/m3.d. Maximum methane production of 0.43 m^/kg VS loaded occurred at two combinations of loading and HRT: 9 d HRT 2.0 kg VS/m3.d and 6 d HRT 4.0 kg VS/m^-d. Methane percentage in the biogas ranged from 82 to 66%. COD reduction ranged from 46 to 97% with the larger reductions occurring at the longer HRTs. Most of the suspended solids were found in the void spaces between the rock media with attached biomass found largely in the 9 d HRT filter. Theoretical application of the anaerobic filter to the treatment of dilute effluent from a settling basin indicated that positive energy budgets may be difficult to obtain due to large heating energy requirements for the filter influent.