Effect Of Shock Loading Research Articles

Effect of rapid hydraulic shock loads on the performance of granular bed baffled reactor

This paper describes the performance of a granular bed baffled reactor when receiving unpredictable hydraulic shock loads, which is a frequent occurrence in industrial wastewater treatment plants. Shock loads were created by rapidly increasing volumetric organic loading rates from 2.5 to 20 kg COD m−3 d−1, by decreasing (in a stepwise fashion) hydraulic retention time from 48 firs to 6 his. Synthetic wastewater containing glucose as the main organic compound was used in this study. High effluent quality was observed, with soluble COD removal efficiencies of 94% to 97%, during all shock loading conditions at steady state. The reactor appeared to possess high tolerance to rapid hydraulic changes with fast recovery time. Marked phase separation between different microorganisms occurred at high organic loading rate, with addogenesis and methanogenesis being the respective dominant activities in the upstream and downstream compartments of the reactor. The compartmentalised nature of the reactor and the granular bed structure were believed to be responsible for high reactor stability during overloading conditions. Microbial ecology in the system appeared to favour acetate and butyrate production at high organic loading rate. Granular biomass possessed good settling characteristics, hence encouraging high biomass retention within the system. The dense granular bed in the methanogenic zone acted as a filtration bed for lighter floating non‐granular biomass, thus further reducing overall effluent solids concentration. This study demonstrated that the granular bed baffled reactor is a suitable system for treating industrial wastewaters with highly varying flow rates

Effect of organic carbon shock loading on endogenous denitrification in sequential batch reactors.

This work was focused on the performance evaluation of sequential batch reactors (SBR) treating sewage, through a process of endogenous biological denitrification. Different operational conditions were carried out, and the behaviour under the effects of organic shock loading was examined. Three laboratory scale reactors were operated simultaneously and fed with similar wastewater. The substratum was molasses and nitrate, as carbon and nitrogen sources, respectively. The three reactors were operated during different aeration periods (0, 15 and 30 min). Sudden changes (shock loading) in organic matter concentration were performed during the experiment. Thus, influent load was quickly increased threefold in relation to the original concentration. Results indicated that SBR reactors withstand adequately moderate shock loading. With regard to substratum degradation, nitrate elimination achieved was approximately 80%, while denitrification rate was approximately 0.87 mgg(-1)h(-1).

Influence of hydraulic shock loads and TDS on the performance of large-scale CETPs treating textile effluents in India

The present study relates to the influence of hydraulic shock loads and total dissolved solids (TDS) on the performance of three large-scale common effluent treatment plants (CETPs) treating textile effluents, which is generated from clusters of small-scale industries in the state of Rajasthan in India. Of the three CETPs, two having capacities 7.0 and 9.0 million liters per day (mld) are located in Pali and one of capacity 6.0 mld in Balotra, District Barmer. Wastewater from about one thousand small-scale industries is treated in these CETPs. The effects of hydraulic shock loads and TDS on effluent data from secondary clarifier for parameters biochemical oxygen demand (BOD) and suspended solids (SS) for CETPs at Pali, and chemical oxygen demand (COD) and SS for the CETP at Balotra were studied. It was observed that the effluent BOD and SS remained within the prescribed limits for CETP Pali at 30% increased flow rate, whereas effluent COD and SS at 30% increased flow rate for CETP Balotra exceeded the prescribed limits and the CETP could sustain 20% increased flow rate. The shock loading analysis revealed that CETP Balotra had reduced capacity to sustain shock loads by 10% as compared to CETP Pali due to the presence of high TDS (15 000–20 000 mg/l). High TDS interfered with the oxygen transfer necessary for biological metabolism, thereby affected the efficiency of activated sludge process. Hence, activated sludge process treating high TDS effluents are more sensitive to hydraulic shock loads and prone to process upsets.

Microstructures in an Fe Based Shape Memory Alloy and Al Based Dual Phase Crystals after High Speed Deformation

Microstructures have been examined after high-speed deformation given by free weight drop and air gun shooting. Special care was exercised to control the amount of plastic strain so that the deformed specimen could be examined as a function of total plastic strain as well as strain rate. Al based dual phase alloys of Al-Al 3 Fe, Al-Al 3 Ti and Al-Al 3 Ni have been examined both at slow and high-speed strain rates using sheet specimens of various thickness, in order to find strain rate effects on the deformability of the ordered second phase particles embedded in Al. The ordered phases, not deformable at a low strain rate in the Al based alloys, are shown to be deformed by dislocation glide in the high strain rate tests. The another aspect of the study is concerned with the mechanical and structural changes in Fe-Mn-Si based shape memory alloys by high-speed deformation. The shape recovery under the opposing stress is found to increase notably after the shape deformation given by shock loading. Also, the effect of shock loading was examined using a thin foil specimen prepared for electron microscopy. In these tests, thin foil regions were locally amorphized presumably by the heavy shear deformation induced at high strain rate.

The effect of heavy metals on the activated sludge process and its microbial community analysis using 16S ribosomal DNA

The effect of heavy metal shock loading on biological treatment systems was studied by traditional methods and molecular biological techniques. Two kinds of SBR (sequence batch reactor) operation units, unacclimated and acclimated activated sludge systems, were studied. The addition of special nutrients and powdered activated carbon (PAC) to stimulate heavy metal uptake and recovery were studied. The kinetic constants could be used to describe the effect of the inhibition of substance utilisation. The results showed that heavy metal shock loading had a greater effect on the unacclimated activated sludge system than on the acclimated one. The special nutrients greatly enhanced the uptake of copper, and the PAC improved sludge settling and decreased the turbidity of the effluent. The variation of dominant species and the diversity of the bacterial community were analysed using 16S ribosomal DNA. Compared with the slight change of dominant species during acclimation by copper, there was a great change in the acclimated system shocked by a high concentration of copper. The results confirmed that the acclimation could improve the resistance of microorganisms to heavy metal toxicity.

Treatment of anaerobically pre-treated domestic sewage by a rotating biological contactor

The performance of a rotating biological contactor (RBC) for the post-treatment of the effluent of an up-flow anaerobic sludge blanket (UASB) was the subject of this study. Different hydraulic and organic loading rates have been investigated. The removal efficiencies of COD total, COD suspended, COD colloidal and COD soluble increased at a higher hydraulic retention time (HRT) and a lower influent organic loading rate. The results obtained indicated that a two-stage RBC reactor at an HRT of 10 h and an organic loading rate of 6.4 g COD m −2 d −1 represents an effective post-treatment process. Most COD suspended and COD colloidal were removed in the first stage while nitrification proceeded in the second stage. The overall nitrification efficiency was 92% at an ammonia loading rate of 1.1 g m −2 d −1. Total E. coli removal at HRTs of 10, 5 and 2.5 h were 99.5%, 99.0% and 89.0%, respectively. The major part of suspended E. coli (>4.4 μm) was removed by sedimentation or by adsorption in the biofilm of the first stage of RBC (99.66%). However, E. coli in the colloidal fraction (<4.4 to >0.45 μm) was eliminated in the second stage of RBC (99.78%). A comparison of the performance of a one-stage versus two-stage RBC system, operated at the same total loading rate, revealed an improvement in the effluent quality of the two-stage effluent as compared to the one-stage effluent. The two stages RBC were used to examine the effect of hydraulic shock loads on reactor performance in terms of COD, nitrification and E. coli removal.

Modeling of Shock Propagation and Attenuation in Viscoelastic Components

Protection from the potentially damaging effects of shock loading is a common design requirement for diverse mechanical structures ranging from shock accelerometers to spacecraft. High damping viscoelastic materials are employed in the design of geometrically complex, impact-absorbent components. Since shock transients are characterized by a broad frequency spectrum, it is imperative to properly model frequency dependence of material behavior over a wide frequency range. The Anelastic Displacement Fields (ADF) method is employed herein to model frequency-dependence within a time-domain finite element framework. Axisymmetric, ADF finite elements are developed and then used to model shock propagation and absorption through viscoelastic structures. The model predictions are verified against longitudinal wave propagation experimental data and theory.

Effect of Carbon Source on PCE Dehalogenation

A laboratory study using the upflow anaerobic sludge blanket reactor for treating high-strength wastewater containing tetrachloroethene (PCE) was carried out to study the effect of carbon source, recycle, and shock loading on dehalogenation of PCE and process performance. The PCE was dehalogenated to trichloroethylene, cis-1,2-dichloroethylene, vinyl chloride, and ethylene. During the study on the effect of carbon source, the PCE and COD removal up to 97% and biogas production of 0.518–0.47 m3/kg CODrem with methane content up to 66% were achieved under steady-state operating conditions. An increase in the influent COD from 2,000 to 4,000 mg/L did not show any improvement in the PCE removal. Recycling of effluent at 50% showed the decrease in COD removal and increase in the effluent concentration of dichloroethylene and vinyl chlorides. Around 1–3.5% of influent PCE stripping to biogas was observed. It was observed that methanol has the stimulatory effect on the dehalogenation of PCE. A shock loading stud...

Nitrogen removal in attached growth waste stabilization ponds of wastewater from a rubber factory

Nitrogen removal from wastewater from rubber factories using attached-growth waste stabilization ponds (AGWSP) was evaluated. Usually, wastewaters generated from rubber factories such as concentrated latex factories and rubber sheet factories contain a high amount of nitrogen originating from natural rubber and ammonia compounds added in the production processes. From an investigation of 3 rubber factories at Songkhla, Thailand, average concentrations of TKN, NH3-N and Org-N in raw factory wastewater were found to be 889, 578 and 311 mg/l respectively. Two series of laboratory waste treatment ponds, waste stabilization ponds (WSP) and attached-growth waste stabilization ponds were investigated to compare the efficiency of nitrogen removal from wastewater from a concentrated latex factory. The wastewater fed to the experimental units was collected from the effluent of the anaerobic treatment pond at the factory. The experiments were conducted with hydraulic retention times (HRT) of 40 and 20 days. Another experiment run with an HRT of 40 days together with 50% recirculating of effluent was also conducted. Finally, an experiment run at an HRT of 4 days was carried out in order to observe the effect of shock loading. The results revealed that TKN, NH3-N and BOD5 removal efficiencies in AGWSP were higher than in control ponds (WSP). Increased removal efficiencies were achieved which resulted from an increase in biomass on media in the pond water.

Nitrogen removal in attached growth waste stabilization ponds of wastewater from a rubber factory

Nitrogen removal from wastewater from rubber factories using attached-growth waste stabilization ponds (AGWSP) was evaluated. Usually, wastewaters generated from rubber factories such as concentrated latex factories and rubber sheet factories contain a high amount of nitrogen originating from natural rubber and ammonia compounds added in the production processes. From an investigation of 3 rubber factories at Songkhla, Thailand, average concentrations of TKN, NH 3 -N and Org-N in raw factory wastewater were found to be 889, 578 and 311 mg/l respectively. Two series of laboratory waste treatment ponds, waste stabilization ponds (WSP) and attached-growth waste stabilization ponds were investigated to compare the efficiency of nitrogen removal from wastewater from a concentrated latex factory. The wastewater fed to the experimental units was collected from the effluent of the anaerobic treatment pond at the factory. The experiments were conducted with hydraulic retention times (HRT) of 40 and 20 days. Another experiment run with an HRT of 40 days together with 50% recirculating of effluent was also conducted. Finally, an experiment run at an HRT of 4 days was carried out in order to observe the effect of shock loading. The results revealed that TKN, NH 3 -N and BOD 5 removal efficiencies in AGWSP were higher than in control ponds (WSP). Increased removal efficiencies were achieved which resulted from an increase in biomass on media in the pond water.

Effects of periods of starvation and fluctuating hydrogen sulfide concentration on biofilter dynamics and performance

The paper describes the results of a systematic study of the transient behavior of biofilters treating reduced sulfur pulping odors and VOCs. They were exposed to variations in contaminant loading and periods of starvation. Three bench-scale biofilters with different filter media were used. Filter media materials used were the mixtures of compost/perlite (4:1), hog fuel/perlite (4:1), and compost/hog fuel/perlite (2:2:1). Hydrogen sulfide, the main malodorous gas produced from kraft pulping processes, was used as the test contaminant. The starvation period comprised of two stages: the `no-contaminant-loading phase' when only humidified air was passing through the biofilters, and the `idle phase' when no air was passing through the biofilters. The response of each biofilter to variations in contaminant mass loading was studied by abruptly changing the concentration and/or flow rate of the inlet waste air stream. Contaminant concentration was continuously measured until a new steady state, for each stage, was achieved. Concentration spikes were applied to study the effects of shock loading on the biofilter removal rates. Biofilters responded effectively to H 2S concentration variations and shock loading by rapidly recovering to the original removal rates within 2–8 h. The re-acclimation times to reach full capacity were very short ranging between 15 and 120 h. Extended periods of starvation resulted in longer re-acclimation periods, so does the idle phase as compared to no-contaminant-loading phase.

Process audit and asset assessment using on-line instrumentation

The traditional approach to assessing the performance of an Activated Sludge Plant (ASP) is to conduct an expensive and often labour intensive through-plant sampling survey. The results from this kind of survey do not give an accurate picture of the plant performance in real time, and often can be ruined by inaccurate or spoilt analytical results. A new approach to Process Audits has been developed based on on-line instrumentation and remote monitoring. The key equipment required to dynamically monitor the ASP performance is the MSL Respirometer which is used to measure organic loading, required retention time and aeration system requirements. Auto-cleaning, auto-calibrating Dissolved Oxygen sensors are used in conjunction with a Respirometer to assess the aeration system efficiency (actual Kla value) and the maximum oxygen demand (OUR) which can be sustained by the plant. Ultrasonic flow meters are used to determine flow splits and actual plant retention time which is compared to the required retention time measured by the Respirometer. Mixed Liquor Suspended Solids (MLSS) profiles are monitored continuously using a self-cleaning optical sensor, which is used along with Respirometric readings to measure the effect of poor wastage regimes and ingress of inert solids. On-line ammonia, phosphate and nitrate monitors can be used to give information on the diurnal variation of nutrients, related to the loading on the plant. As well as the normal plant operation, the effect of shock loads, storm conditions and sludge liquor returns can all be assessed. All instruments are linked via a serial communications loop to a modern and telephone line. The plant is monitored remotely, making it an unobtrusive survey allowing operations to continue as normal and giving a true picture of the plant performance. This approach has been used on several UK sewage treatment works and has proven to be a cost-effective method. In one case a proposed capital spend on final settlement tanks was shown to be unnecessary; in another case the plant was shown to be unable to support a nitrifying biomass.

Process audit and asset assessment using on-line instrumentation

The traditional approach to assessing the performance of an Activated Sludge Plant (ASP) is to conduct an expensive and often labour intensive through-plant sampling survey. The results from this kind of survey do not give an accurate picture of the plant performance in real time, and often can be ruined by inaccurate or spoilt analytical results.A new approach to Process Audits has been developed based on on-line instrumentation and remote monitoring. The key equipment required to dynamically monitor the ASP performance is the MSL Respirometer which is used to measure organic loading, required retention time and aeration system requirements.Auto-cleaning, auto-calibrating Dissolved Oxygen sensors are used in conjunction with a Respirometer to assess the aeration system efficiency (actual Kla value) and the maximum oxygen demand (OUR) which can be sustained by the plant. Ultrasonic flow meters are used to determine flow splits and actual plant retention time which is compared to the required retention time measured by the Respirometer. Mixed Liquor Suspended Solids (MLSS) profiles are monitored continuously using a self-cleaning optical sensor, which is used along with Respirometric readings to measure the effect of poor wastage regimes and ingress of inert solids. On-line ammonia, phosphate and nitrate monitors can be used to give information on the diurnal variation of nutrients, related to the loading on the plant. As well as the normal plant operation, the effect of shock loads, storm conditions and sludge liquor returns can all be assessed.All instruments are linked via a serial communications loop to a modem and telephone line. The plant is monitored remotely, making it an unobtrusive survey allowing operations to continue as normal and giving a true picture of the plant performance.This approach has been used on several UK sewage treatment works and has proven to be a cost-effective method. In one case a proposed capital spend on final settlement tanks was shown to be unnecessary; in another case the plant was shown to be unable to support a nitrifying biomass.

Dissolved hydrogen concentration as an on-line control parameter for the automated operation and optimization of anaerobic digesters

The use of dissolved hydrogen as an early warning signal of digester failure and a control parameter to operate anaerobic digesters was investigated. A sensitive, on-line method was developed for measuring trace levels of dissolved hydrogen in a semi-permeable membrane, situated within the biomass of a 1 L laboratory anaerobic digester, using trace reduction gas analysis. At normal operating conditions, the dissolved hydrogen partial pressure (2 to 8 Pa) was found to be linearly correlated with the loading rate of the digester, and was a sensitive indicator of the effect of shockloads as well as gradual overloading. An increase in hydrogen partial pressure above a critical concentration of 6.5-7 Pa indicated the initial stage of digester overloading (i.e., volatile fatty acids accumulation). A H(2)-based computer control system, using a critical hydrogen partial pressure of 6.5 Pa as the setpoint, was found to be effective for the safe operation of a laboratory digester close to its maximum sustainable loading rate. The existence of a relationship between hydrogen level and organic loading rate was also confirmed on a 600 m(3) industrial digester, with digester overloading occurring at hydrogen concentrations above 7 Pa. The results suggest that the dissolved hydrogen concentration is capable of being a sensitive on-line parameter for the automated management of anaerobic digesters near their maximum sustainable loading capacity. (c) 1997 John Wiley & Sons, Inc. Biotechnol Bioeng 56: 626-634, 1997.

Effects of heavy metals on nitrifying bacteria

Laboratory evaluations were conducted to study the toxic responses of heavy metals such as copper and nickel of an autotrophic culture of strict nitrifiers (Nitrosomonas sp. and Nitrobacter sp.) in continuous flow stirred tank reactors (CSTR). One of the CSTRs was operated as a suspended growth (SG) system while the other was operated as an attached &amp; suspended growth (A&amp;SG) system. Nitrification inhibition in a SG and A&amp;SG systems was investigated with the emphasis on the effect of shock loading of copper and nickel. As a result of the copper and nickel test, Nitrosomonas sp. was found to be equally or more sensitive than Nitrobacter sp. However, a higher influent nickel concentration of 50 mg/L was needed to cause a similar percent inhibition of ammonium oxidation than the copper concentration of 5 mg/L. A geochemical equilibrium speciation model, MINTEQA2/PRODEFA2, was used to compute the concentrations of various chemical species present in the wastewater for both systems. The high correlations of Cu(NH3)4+2 and Ni(NH3)4+2 with percent inhibition were found and it was thought that they were probably the species responsible for the inhibition of ammonia oxidation.

The effect of shock loads on the performance of an anaerobic baffled reactor (ABR). 1. Step changes in feed concentration at constant retention time

A 10-litre anaerobic baffled reactor (ABR), with eight compartments, was used to examine the effect of organic shock loads, in the form of a step change in the feed chemical oxygen demand (COD) at constant hydraulic retention time (HRT), on reactor performance in terms of COD removal, and to obtain a greater insight into microbial responses and interactions during these shocks. In order to minimise feed variations, and have a totally biodegradable substrate, a synthetic carbohydrate (sucrose)-protein (meat extract) substrate was used. The reactor was operated at 20-h HRT, 4 g/litre COD (4.8 kg-COD/m 3 d), and 35°C for 1 month as a base-line condition, and this resulted in 98% COD removal. It was found that a step change in the feed to 8 g/litre COD (9.6 kg-COD/m 3 d) at 20-h HRT for 20 days did not affect the substrate removal efficiency at all; however, when the concentration was increased to 15 g/litre COD (18 kg-COD/m 3 d) from 4 g/litre for 20 days, the removal efficiency decreased to 90%. It was found that the compartmentalised ABR consisted of three general zones (acidification, methanation, and a buffer zone where little acidification and methanogenesis occurs), and the function of high solids concentrations in the reactor was to enhance stability rather than improve COD removal. Hence, the structure of the ABR prevents most of the biomass being exposed to low pHs during shock loads, and enhances reactor stability. In addition, due to low pHs and high substrate concentrations in the first compartment, a microbial population seems to be selected which produces primarily acetate and butyrate rather than formate and propionate, and this also enhances the stability of the reactor during shock loads. In contrast to past results, formate did not seem to be an important interspecies electron carrier except under high mixing conditions and shock loads, and this was postulated to be due to differences in the structure of the microbial flocs. Based on these observations, the ABR holds some promise as a reactor design for anaerobic industrial wastewater treatment.

The effect of shock loads on the performance of an anaerobic baffled reactor (ABR). 2. Step and transient hydraulic shocks at constant feed strength

Two Anaerobic Baffled Reactors (ABR) were used to examine the effect of transient and step hydraulic shock loads on reactor performance in terms of chemical oxygen demand (COD) removal and microbial responses to hydraulic shocks in each compartment. The reactors were operated at 20-h hydraulic retention time (HRT), 4 g/litre COD and 35°C as a base-line condition. Hydraulic shocks with an HRT of 1 h (an increase of 20 times in the influent flowrate), 10 h and 5 h were applied to the reactors for 3 h, 2 weeks and 3.5 weeks, respectively, and a variety of key intermediates monitored over time in each compartment. The baseline conditions (4.8 kg-COD/m 3 d) resulted in 98% COD removal; however, when the HRT decreased to 10 h (9.6 kg-COD/m 3 d) removal dropped to 90%, and at 5-h HRT (19.2 kg-COD/m 3 d) removal was only 52%. It was found that the ABR was very stable to large transient shocks, and while biomass loss was substantial, it recovered back to its basline performance only 9 h after the shock ceased. Tracer studies found that the total dead space in the reactor at 24 g-VS/litre and 20-h HRT was about 18% by volume; however, when the HRT was reduced to 10 and 5 h, this increased to 39%. Comparison of this data with previous results for varying feed concentration and recycle ratio in the ABR revealed that removal efficiency could be tentatively explained in terms of HRT; organic loading rate; the concentration/activity and kinetics of the biomass; the dead space in the reactor; the severity and type of channelling in each compartment; the substrate concentration in each compartment driving mass transfer into the flocs; and floc size. Hence, it seems that this type of reactor configuration has potential in treating industrial wastes that vary in both flow and concentration, and still enable high removal rates to be achieved.

Shock-loading effects on superconductivity of HgBa 2CuO y

We have investigated shock-loading effects on the structural and superconducting properties of a high- T c superconducting oxide HgBa 2CuO y (Hg-1201) by X-ray powder diffraction analysis, magnetic measurements and electron microscopy. Most of the shock experiments were performed using samples with pre-cooling to liquid nitrogen temperature in order to prevent decomposition due to temperature rise. The width of magnetization hysteresis ( ΔM) of Hg-1201 was reduced by shock-loading, but it recovered and improved by subsequent appropriate annealing. The magnetization hysteresis loops and TEM observation indicate that this improvement of ΔM is probably caused by defects as pinning centers which were introduced by shock-loading and preserved by the annealing. The present study demonstrates that the pre-cooled shock-loading method is effective for introducing pinning centers into high- T c superconductors, even for the composition including a volatile element such as Hg.

Effects of hydraulic shock loads on small on-site sewage treatment unit

This research was to determine the effects of hydraulic shock loads by intermittent feeding on the removal efficiency of a small prefabricated real-sized septic tank/anaerobic filter unit. Besides the ‘control’ 24-hour continuous runs the other twice a day feeding periods covered 16, 12 and 8 hours per day, with the tank's retention time varying from 22.5 to 90 hours. It was demonstrated that the variation of the hydraulic feeding patterns ranging from 24-hour continuous feed to 8 h per day intermittent feed did not affect the performance of the system much. The percentage of BOD reduction was decreased from 85 to 82 and 81 percent for the feed times of 24, 16 and 8 hours per day, respectively. The tank capacity played a relatively major role in determining the removal efficiency of the process. The efficiency of BOD and SS reduction dwindled from 82.2 to 68.9 and 56.0% with the corresponding reduction in tank sizes from 90 to 45 and 22.5 hours retention, respectively. In any event, the unit could not satisfactorily remove nutrients. A retention period of not less than 48 hours is recommended if the Thai effluent standards are to be met. Equations for predicting the BOD removal capability were also developed.

Effects of hydraulic shock loads on small on-site sewage treatment unit

This research was to determine the effects of hydraulic shock loads by intermittent feeding on the removal efficiency of a small prefabricated real-sized septic tank/anaerobic filter unit. Besides the ‘control’ 24-hour continuous runs the other twice a day feeding periods covered 16, 12 and 8 hours per day, with the tank's retention time varying from 22.5 to 90 hours. It was demonstrated that the variation of the hydraulic feeding patterns ranging from 24-hour continuous feed to 8 h per day intermittent feed did not affect the performance of the system much. The percentage of BOD reduction was decreased from 85 to 82 and 81 percent for the feed times of 24, 16 and 8 hours per day, respectively.The tank capacity played a relatively major role in determining the removal efficiency of the process. The efficiency of BOD and SS reduction dwindled from 82.2 to 68.9 and 56.0% with the corresponding reduction in tank sizes from 90 to 45 and 22.5 hours retention, respectively. In any event, the unit could not satisfactorily remove nutrients. A retention period of not less than 48 howl is recommended if the Thai effluent standards are to be met. Equations for predicting the BOD removal capability were also developed.