Protozoa of a rotating biological contactor treatment plant in Mexico

A protozoological analysis of the biofilm developed on the discs of an RBC unit was performed using light microscopy; the species found belonged to 2 phyla: Sarcomastigophora with 23, and Ciliophora with 30 species. All isolates were free-living with the exception of Tritrichomonas fecalis. Most abundant species were: Euglena gracilis, Paramecium multimicronucleatum, Tetrahymena pyriformis, Polytoma uvella, Didinium nasutum, Bodo caudatus, Vorticella microstoma, Cyclidium glaucoma, Sathrophilus agitatus, Antophysa vegetans, and Urothrica farcta. Ciliates predominated over the flagellates in number and diversity. According to their locomotion most abundant species were free-swimming, followed by the attached, and crawling forms. In relation to their nutrition most frequent species were bacterivorous, saprozoic, and carnivorous. Values of the saprobic index calculated for each sampling station indicated that α-mesosaprobic conditions prevailed along the contactor with a zone of overlap with polysaprobic conditions at station IV. The RBC received an influent flow that varied from 1.5 to 13.5 L s^?1 and a soluble BOD₅ which range from 46 to 170 mg L^?1. The contactor studied showed an overall BOD₅ removal of 86% and a COD removal of 74%. DO levels influenced the values obtained for the saprobic index and the kind of protozoan populations and communities present. The biological findings (indicator species of the saprobic system), clearly reflected and agreed with the physicochemical results obtained simultaneously; the results showed that the system performed efficiently.     

ROLE OF SUPPLEMENTAL AERATION IN IMPROVING OVERLOADED FIRST-STAGE RBC PERFORMANCE

A study was conducted to evaluate the effectiveness of supplemental aeration in improving the performance of an rotating biological contactor (RBC) treatment system whose first and second stages were overloaded resulting in very low dissolved oxygen conditions and heavy beggiatoa growth thereby deteriorating process performance. A four-stage RBC having two parallel trains and treating combined municipal and industrial dairy wastewater was used to conduct the research. One train was provided with supplemental air in all stages and the other train, without supplemental air, was used as a control. Daily 24-hour compositie samples were collected at the influent to the RBC and at the end of each stage. Dissolved oxygen, pH, and temperature levels were measured in each stage. Samples were analyzed for soluble COD, ammonia nitrogen, total and volatile suspended solids. Samples were also analyzed for soluble BOD₅ and oxygen uptake rate once a week. The biomass thickness on the discs was measured and growth conditions were noted. The experimental results indicate that RBC units with supplemental aeration demonstrated remarkable performance and ability to adapt to differing organic loading rates. In addition, beggiatoa growth was completely eliminated with supplemental air thereby establishing thinner and active aerobic biomass.     

Biochemical Oxygen Demand Relationships in Typical Agricultural Effluents

Understanding the variables controlling biochemical oxygen demand (BOD) of effluents from agricultural systems is essential for predicting and managing the water quality risks associated with agricultural production. In this study, short- and long-term oxygen demand behaviors of waters from primarily agricultural sources and their relationships with other parameters were evaluated. A total of 46 water samples were generated from diverse organic sources commonly associated with agricultural activities and analyzed for BOD and other various water quality parameters. Short-term BOD (BOD₂ and BOD₅) were significantly correlated with total organic carbon (TOC), particulate organic carbon (POC), and dissolved organic carbon (DOC) (R ²?=?0.62–0.77, p?<?0.001), likewise to total nitrogen, total Kjeldahl nitrogen, and nitrite–nitrogen (NO₂–N) (R ²?=?0.40–0.55, p?<?0.001). Long-term BOD (BOD₆₀) was generally poorly correlated with these C and N fractions. Phosphate (PO₄–P) exhibited a positive and linear relationship with both short- and long-term BOD, whereas chloride (Cl) tended to inhibit oxygen demand. Multivariate combinations of each of TOC, POC, and DOC with NO₂–N, and Cl or PO₄–P improved the predictions of both short- and long-term BOD. The ultimate BOD (BOD_u) derived from the first-order kinetics was highly correlated with BOD₆₀ (R ²?=?0.81, p?<?0.001) whereas BOD₆₀ was correlated with BOD₅ (R ²?=?0.60, p?<?0.001). Overall the results indicated that C and N forms along with PO₄–P and Cl were the dominant factors controlling the oxygen demand behaviors of agricultural effluents.     

Oxygen Transfer and Industrial Wastewater Treatment Efficiency of a Vertically Moving Biofilm System

A vertically moving biofilm system (VMBS) was developed to treat wastewater. In this system, the biofilm grows on a biofilm module consisting of plastic media that is vertically and repeatedly moved up into the air and down into the water. The objectives of this study were to investigate the oxygen transfer efficiency and industrial wastewater treatment performance of the VMBS. The oxygen transfer coefficient (K _L a) depended on the movement frequency (n) of the biofilm module and was proportional to n ^1.67. K _L a values measured were within the range of 0.0001 to 0.0027 s^-1. The VMBS exhibited good carbonaceous removal when treating industrial wastewater produced in a factory manufacturing synthetic fibres. Removal efficiency of filtered chemical oxygen demand (COD) and biological oxygen demand (BOD₅) was up to 93.2 and 97.9%, respectively. The volumetric removal rates of filtered COD and BOD₅ reached 1320 g COD m^-3 day^-1 and 700 g BOD₅ m^-3 day^-1. The areal organic removal rates, based on the surface area of the biofilm substrata, were 16 g BOD₅ m^-2 day^-1 and 39 g COD m^-2 day^-1. No clogging occurred during the experiment. The mean areal biofilm mass increased with increasing the mean areal BOD₅ removal rate. The new biofilm process has such advantages as high carbonaceous oxidation, energy saving, simpleconstruction and easy operation for industrial wastewater treatment.     

THE SEASONAL CHANGES OF KERATINOLYTIC FUNGI IN SEDIMENTS OF CATALONIAN RIVERS (SPAIN)

The Catalonian river sediments were found to be rich in keratinolytic fungi. The keratinolytic fungal populations showed clear seasonal changes in the river sediments. The main factors ‘regulating’ these populations in such habitat were temperature, dissolved oxygen concentration, pH, ammonium, nitrates, total fungal number, BOD₅, water poisons (cyanides, detergents, phenols), salinity and, presumably, strong insolation associated with low water levels. The last was probably of special importance in the deterioration of the fungal populations in the spring/summer season. A toxic effect on keratinolytic fungi in sediments was observed.Chrysosporium keratinophilum was found to be most resistant to industrial contaminants and salinity. Therefore, this species could be used as an indicator of water pollution.     

Utilization of created wetlands to upgrade small municipal wastewater treatment systems

Created wetlands offer a low cost, low maintenance, and practical alternative for upgrading secondary municipal wastewater treatment systems. The removal efficiencies, effects of seasonal temperature variations, and effects of increased loading rates on contaminant removal within such a system was studied by Auburn University researchers at a created wetland site in Hurtsboro, Alabama. The 0.16 ha system consisted of a two cell wetlands planted with cattails (Typha latifolia), bulrush (Scirpus validus), arrow duck potatoes (Sagitaria latifolis), burr reeds (Spargaminum eurycarpun), water pennywort (Hydrocotyl ranunculoides), and parrotfeather (Myriophyllum brasiliense). Testing occurred from January through September of 1988 at hydraulic loading rates of 169, 289, and 345 m³ ha^?1 d^?1. The monthly average total suspended solids influent: effluent mg L^?1 concentration ratio during the study period was 135:19 while the monthly average total BOD₅ influent: effluent mg L^?1 concentration ratio was 38:8. Once the system stabilized, the monthly average total BOD₅ effluent concentration remained essentially constant over the range of average BOD₅ loading rates employed in this study. Total Kjeldahl N removal was more effective at loading rates of 2.6 kg ha^?1 d^?1. The monthly average influent: effluent TKN mg L^?1 concentration ratio was 15:4.     

A System Dynamic Model and Sensitivity Analysis for Simulating Domestic Pollution Removal in a Free-Water Surface Constructed Wetland

This work develops a system dynamic simulation model for free-water surface constructed wetlands, as well as provides appropriate values for the parameters of constructed wetland management. The system dynamic model is calibrated and validated by using data from a 1-year study of a constructed wetland in Tainan of southern Taiwan. Additionally, the major parameters that affect the simulation output are obtained via sensitivity analysis by using generalized likelihood uncertainty estimation (GLUE). A high R ² and Nash?CSutcliffe coefficient of efficiency between the simulated and measured outflow values indicate that in addition to reproducing the changing trends of dissolved oxygen (DO), 5-day biological oxygen demand (BOD₅), total nitrogen (TN), total suspended sediment (TSS), and total phosphorous (TP) concentrations, the model can simulate the variations of DO, BOD₅, and TSS. Taken into account the interactions among parameters, the GLUE method successfully obtained the model sensitive parameters from the Monte Carlo parameter sets. Sensitivity analysis results indicate that the parameters of microorganisms are sensitive factors that affect DO, BOD₅, and TN, while sediment diameter largely influences TP and TSS. Further elucidating environmental microorganisms would increase the model accuracy and provide a valuable reference for constructed wetland management and design.     

Soil biological properties of a montane forest sere: Corroboration of Odum's postulates

Changes in organic C content, N component pools, Shannon-Weaver diversity index (H′) of microbial populations, nitrification potential, and ATP and dehydrogenase activities were examined in soils along a montane meadow-aspen-fir-spruce sere.Along the sere organic C increased from 2.15 to 26.8%, total N from 0.13 to 0.98%, C: N ratios from 17 to 27, total NH₄⁺ from 103 to 850 μg g^?1, total NH₄⁺:NO₃^? ratios from 69 to 326, and microbial diversity index, H′, from 0.87 to 1.28. Coefficients of determination, r², for H′ vs organic C and total N in A-horizons, were 0.99 and 0.98, respectively, and H′ vs combined O- and A-horizons 0.68 and 0.70, respectively, indicating the presence of different microbial communities in the mineral and forest floor soils. Radiocarbon dating of humic acids and humin showed the longest mean residence times (920 and 1050 yr BP) in the meadow soils, suggesting a more efficient organic matter turnover and selective accumulation of recalcitrant organic components than in soils of more mature stages. The ATP content and dehydrogenase activity values were not statistically different in the forest sequence soils. Rates of nitrification potentials measured in vitro increased along the sere in the surface soils.Information obtained from seral soil variables supported hypothesized successional trends relating to organic matter content, species diversity, nutrient cycling, nutrient exchange rate and nutrient conservation. Nitrification potentials of soils, however, contradicted the postulate that nutrient conservation increases as an ecosystem matures.     

Soil nematode assemblages as bioindicators of primary succession along a 120-year-old chronosequence on the Hailuogou Glacier forefield,SW China

Successional dynamics in terrestrial ecosystems is important for interactions between aboveground and belowground subsystems. In this study, nematode communities in a Hailuogou Glacier Chronosequence from seven stages were investigated to determine whether changes in soil phosphorus (P) and nematode assemblages parallel those observed in aboveground communities, and whether the primary succession in this chronosequence has entered a retrogressive phase after 120 years of succession. The initial 40-year succession, including stages 2, 3 and 4, can be viewed as a build-up phase. Especially at stage 3, vegetation succession from grassland to forest accelerated the accumulation of plant litter and bioavailable P, paralleled with a sharp increase in nematode abundance. The mature phases covering stages 5, 6 and 7 displayed most balanced nematode communities, in which abundance, taxon richness, maturity index and structure index were at highest. However, the last stage 7 appeared to show some retrogressive characteristics, as suggested by the reduced bioavailability of P and a significant decrease in nematode densities, along with the disappearance of some rare genera of nematodes from higher trophic guilds, resulting in decreases in the nematode channel ratio, plant parasite index and enrichment index. Thus, the Hailuogou Glacier Chronosequence may enter its retrogressive phase during the next decade or century. A bacterial-based nematode energy channel dominated the chronosequence during the development; by contrast, a fungivore-based channel was activated at the early and late stages, because fungivores are better adapted to nutrient-poor environments. Our results demonstrated that different nematode guilds have contrasting responses to chronosequence stages, possibly due to their different responses to bottom-up and top-down controls. Furthermore, soil nematode communities could be used as sensitive bioindicators of soil health in glacial-retreat areas.     

The Effects of Hydraulic and Organic Loadings on the Performance of a Full-Scale Facultative Pond in a Temperate Climate Region (Argentine Patagonia)

This work focuses on the performance of a primary facultative pond, in a full-scale waste stabilization pond system, located in a temperate climate region (average air temperature in winter, 7.7°C; spring and autumn, 14.0°C; and summer, 19.9°C) in Puerto Madryn city??Argentine Patagonia (42°45??S; 65°05??W). Experimental work was conducted for 43?months in seven sampling points. During the experimental time frame, the influent flow rate increased from 12,000 to 15,500?m³/day; the surface organic loading ranged from 55 to 68?kg BOD₅/ha·day and the theoretical retention time decreased from 31 to 24?days. The results indicate that a primary facultative pond performing in this region, to keep predominant facultative conditions and acceptable filtered biochemical oxygen demand (BOD₅) removal, should be loaded with an organic loading rate of up to 60?kg BOD₅/ha·day. The flow and organic loading increase affected the ammonium removal process, extending the period time in which ammonium removal was less than 50% and nitrate was not detectable; at first, this period occurred during winter strictly and then covered part of autumn and part of spring, too. Ammonium removal was clearly temperature dependent and directly related to chlorophyll a and nitrate concentrations (i.e. higher ammonia removals were reported under summer conditions when chlorophyll a and nitrate concentrations were higher), but was not linked with high pH values. The ammonium volatilization as a predominant removal process could be discarded, while ammonium nitrification?Cdenitrification and algal nitrogen uptake seems to be the dominant mechanisms.     

Soil crust microfungi along a southward rainfall gradient in desert ecosystems

We examined mycobiota structure, diversity level, and density of microfungal isolates in the biological soil crusts (BSC) and non-crusted soil of the northern and central Negev desert in 10 locations along a southward rainfall gradient. A total of 87 species from 49 genera were isolated. The mycobiota of BSC (80 species) was characterized by dominance of melanin-containing fungi, the remarkable contribution of sexual ascomycete species, and low abundance of the genera Penicillium and Aspergillus. Species richness and the contribution of dark-colored fungi with large, multicellular spores negatively and significantly correlated with rainfall. The comparison with mycobiota of the non-crusted soil showed that in BSC communities, the xeric “desert” component (melanics) was significantly more pronounced, and the mesic “forest” component (Penicillium) was much less represented. In addition, the weak influence of rainfall on spatial variations of most observed mycobiotic characteristics indicated that microenvironmental factors (soil moisture, temperature, organic matter content) influenced the development of studied communities more essentially than macroenvironmental (climatic) factors.     

A Novel High Capacity Biofilm Reactor System for Treatment of Domestic Sewage

A lab-scale novel biofilm reactor system, Ultra-Compact Biofilm Reactor (UCBR), was studied to investigate its performance and operational characteristics for domestic sewage treatment. The reactor was operated at four different hydraulic retention times, namely, 90, 60, 30 and 15 min. The operating ranges of volumetric loading rates in terms of COD, BOD₅, NH⁺ ₄-N and TKN were 5.6-62.1 kg COD/m³ d, 2.6-32.5 BOD₅/m³ d, 0.6-3.2 kg NH⁺ ₄-N/m³ d and 0.82-6.2 kg TKN/m³ d, respectively. The COD, BOD₅ and NH⁺ ₄-N removal efficiencies at 90-min hydraulic retention time (HRT) and 60-min HRT could exceed 80%, 90% and 99%, respectively. The corresponding maximum biomass concentrations were 12.0 g/L and 15.0 g/L at 90-min HRT and 60-min HRT, respectively. At 30-min HRT, the biomass concentration increased to a maximum of 24.0 g/L. However, COD and BOD₅ removal efficiencies decreased to 75% and 80%, respectively, while the NH⁺ ₄-N nitrification efficiency decreased to only 25% to 30%. These observations suggested that high biomass concentration alone was not sufficient to provide a high removal capacity in a UCBR. Further reduction in HRT to 15 min led to an excessive biomass decline from 22.5 g/L to 4.0 g/L. On the whole, the UCBR was able to sustain COD removal and NH⁺ ₄-N conversion of up to 5.96-18.70 kg COD/m³ d and 0.73-1.00 kg NH⁺ ₄-N/m³ d, respectively.     

养殖水中总有机碳与化学需氧量和生化需氧量相关性的研究

总有机碳能较全面地反映水体有机物的污染程度,总有机碳的测定具有操作简单、分析快速、结果准确等特点。研究了养殖水中的总有机碳与化学需氧量和生化需氧量相关性,总有机碳与化学需氧量的相关系数为０.９９１８,总有机碳与生化需氧量的相关系数为０.９３６２,可以用总有机碳来估算和衡量水体的污染情况。     

Optimization of Petroleum Refinery Wastewater Treatment by Vertical Flow Constructed Wetlands Under Tropical Conditions: Plant Species Selection and Polishing by a Horizontal Flow Constructed Wetland

Typha latifolia-planted vertical subsurface flow constructed wetlands (VSSF CWs) can be used to treat petroleum refinery wastewater. This study evaluated if the removal efficiency of VSSF CWs can be improved by changing the plant species or coupling horizontal subsurface flow constructed wetlands (HSSF CWs) to the VSSF CW systems. The VSSF CWs had a removal efficiency of 76% for biological oxygen demand (BOD₅), 73% for chemical oxygen demand (COD), 70% for ammonium-N (NH₄⁺-N), 68% for nitrate-N (NO₃^?-N), 49% for phosphate (PO₄^3?-P), 68% for total suspended solids (TSS), and 89% for turbidity. The HSSF CWs planted with T. latifolia further reduced the contaminant load of the VSSF CW-treated effluent, giving an additional removal efficiency of 74, 65, 43, 65, 58, 50, and 75% for, respectively, BOD₅, COD, NH₄⁺-N, NO₃^?-N, PO₄^3?-P, TSS, and turbidity. The combined hybrid CW showed, therefore, an improved effluent quality with overall removal efficiencies of, respectively, 94% for BOD₅, 88% for COD, 84% for NH₄⁺-N, 89% for NO₃^?-N, 78% for PO₄^3?-P, 85% for TSS, and 97% for turbidity. T. latifolia strived well in the VSSF and HSSF CWs, which may have contributed to the high NH₄ ⁺-N, NO₃^?-N, and PO₄^3?-P removal efficiencies. T. latifolia-planted VSSF CWs showed a higher contaminant removal efficiency compared to the unplanted VSSF CW. T. latifolia is thus a suitable plant species for treatment of secondary refinery wastewater. Also a T. latifolia-planted hybrid CW is a viable alternative for the treatment of secondary refinery wastewater under the prevailing climatic conditions in Nigeria.     

Efficiency of Bark, Activated Charcoal, Foam and Sand Filters in Reducing Pollutants from Greywater

Greywater is a potential resource of water that can be improved to meet the quality needed for irrigation. This study evaluated the performance of bark, activated charcoal, polyurethane foam and sand filters in removing biochemical oxygen demand (BOD₅), surfactants, phosphorus, nitrogen and microbial indicators from greywater during start-up and steady state. In column experiments, 0.6?m high filters (diameter 20?cm) were fed for 113?days with artificial greywater at a hydraulic loading rate of 0.032?m³?m^?2?day^?1 and an organic loading rate of 0.014?kg BOD₅ m^?2?day^?1. Bark and activated charcoal efficiently reduced the concentrations of organics (BOD₅), surfactants (methylene blue active substances??MBAS), total phosphorus (Tot-P) and total thermotolerant coliform numbers, while sand and foam were less efficient. Bark, activated charcoal, foam and sand reduced influent BOD₅ by 98, 97, 37 and 75?%; MBAS by >99, >99, 73 and 96?%; Tot-P by 97, 91, 36 and 78?%; and total nitrogen by 19, 98, 13 and 5?%, respectively. BOD₅ and MBAS were efficiently reduced directly from start-up by bark and activated charcoal, while foam needed 30?days to achieve about 50?% reduction in BOD₅. Bark was the most efficient filter in reducing thermotolerant faecal coliforms (2.4 log₁₀), while foam achieved the lowest reduction (0.5 log₁₀). Overall, bark and activated charcoal filters appeared to be the most suitable filters for improving greywater quality to reach irrigation quality in terms of organic matter reduction. Performance of these filters under higher and fluctuating loadings and the long-term sustainability of the filter materials need further investigation.     

BOD/TOC correlations and their application to water quality evaluation

The error residual for TOC analysis is examined with the result that two BOD₅/TOC models are suggested, both of which use a multiplicative error structure. A comparison of these models with the historically-assumed linear BOD₅/TOC model is created for data collected from the City of Waterloo's Water Pollution Control Plant. The predictive capabilities of the derived BOD₅/TOC models are examined for interpolation and extrapolation potentials for augmenting water quality data information bases. Use of the models to reduce the statistical uncertainty associated with BOD measurements is considered.     

Root zone microbial populations, urease activities, and purification efficiency for a constructed wetland

In order to investigate the effects of microorganisms and their urease activities in macrophytic root zones on pollutant removal, four small-scale plots (SSPs) of vertical/reverse-vertical flow wetlands were set up to determine: a) the relationship between the abundance of microorganisms in the root zones and water purification efficiency; and b) the relationship between urease activities in the root zones and pollutant removal in a constructed wetland system. Total numbers of the microbial population (bacteria, fungi, and actinomyces) along with urease activities in the macrophytic root zones were determined. In addition, the relationships between microbial populations and urease activities as well as the wastewater purification efficiencies of total phosphorus (TP), total Kjeldahl nitrogen (TKN), biochemical oxygen demand in 5 days (BOD₅), and chemical oxygen demand (COD) were also analyzed. The results showed that there was a highly significant positive correlation (r = 0.9772, P < 0.01) between the number of bacteria in the root zones and BOD₅ removal efficiency and a significant negative correlation (r = -0.9092, P < 0.05) between the number of fungi and the removal efficiency of TKN. Meanwhile, there was a significant positive correlation (r = 0.8830, P < 0.05) between urease activities in the root zones and the removal efficiency of TKN. Thus, during wastewater treatment in a constructed wetland system, microorganism and urease activities in the root zones were very important factors.     

Bacterial communities in soil mimic patterns of vegetative succession and ecosystem climax but are resilient to change between seasons

Organism succession during ecosystem development has been researched for aboveground plant communities, however, the associated patterns of change in below-ground microbial communities are less described. In 2008, a study was initiated along a developmental sand-dune soil chronosequence bordering northern Lake Michigan near Wilderness Park (WP). It was hypothesized that soil bacterial communities would follow a pattern of change that is associated with soil, plant, and ecosystem development. This study included 5 replicate sites along 9 soils (n = 45) ranging in age from ∼105 to 4010 years since deposition. Soil bacterial community composition and diversity were studied using bacterial tag-encoded FLX amplicon pyrosequencing of the 16S rRNA gene. Bray–Curtis ordination indicated that bacterial community assembly changed along the developmental soil and plant gradient. The changes were not affected by seasonal differences, despite likely differences in plant root C (e.g. exudates), temperature, and water availability in soil. Soil base cations (Ca, Mg) and pH declined, showing log-linear correlations with soil age (r ∼ 0.83, 0.84 and 0.81; P < 0.01). Bacterial diversity (Simpson's 1/D) declined rapidly during the initial stages of soil development (∼105–450 y) and thereafter (>450 y) did not change. Turnover of plant taxa was also more rapid early during ecosystem development and correlated with bacterial community structural change (P < 0.000001; r = 0.56). It is hypothesized that plants help to drive pedogenic change during early (<450 y) soil development (e.g. pH decline, cation leaching) which drive selection of soil bacterial communities. In mature soils (∼450–4000 y), resilient and stable soil bacterial community structures developed, mimicking steady-state climax communities that were observed during latter stages of primary plant succession. These relationships point to possible feedbacks between plant and bacterial communities during ecosystem development.     

Microbial degradation of synthetic sperm whale oils

Thirteen synthetic sperm whale oils, including four sulfated and three sulfurized products, were tested for biodegradability using growth ofPseudomonas aeruginosa and BOD₅ as indicators. All of the underivatized oils were found to be easily degraded; the sulfated and sulfurized products were relatively resistant. Four oils showed anti-microbial properties at concentrations of more than 1.0%. All the replacements are capable of causing BOD loading problems.     

Ammonia Nitrogen Removal in Step-Feed Rotating Biological Contactors

The effect of step-feed on biological ammonia nitrogen (NH₃-N) removal in a rotating biological contactor (RBC) system consisting of two three-stage units (one control and one step-feed) treating synthetic wastewater was examined. The performance of the step-feedRBC was evaluated in comparison to a regularly fed RBC in terms of NH₃-N removals and stage-dissolved oxygen (DO) conditions over a range of hydraulic and organic loading rates (HLR = 0.032 to 0.125 m³ m^-2. d^-1 and OLR = 11.03 to 111.6 g COD m^-2. d^-1). The results indicate that the step-feed unit showed better removal efficiency (%) at high HLR and ORL than the regularly fed control unit. Increasing the HLR and the OLR resulted in a decrease in DO in all the stages of the two units. However, DO values in the step-feed system were higher than those recorded for the control system. In addition, O₂ limiting conditions (DO ≤ 2 mg l^-1) and heavy growth of Beggiatoa were detected in stage 1 of the control unit at high loading rates.