A review of modeling approaches in activated sludge systems

The feasibility of using models to understand processes, predict and/or simulate, control, monitor and optimize WasteWater Treatment Plants (WWTPs) has been explored by a number of researchers. Mathematical modeling provides a powerful tool for design, operational assistance, forecast future behavior and control. A good model not only elucidates a better understanding of the complicated biological and chemical fundamentals but is also essential for process design, process start-up, dynamics predictions, process control and process optimization. This paper reviews developments and the application of different modeling approaches to wastewater treatment plants, especially activated sludge systems and processes therein in the last decade. In addition, we present an opinion on the wider wastewater treatment related research issues that need to be addressed through modeling.Key words: Mathematical modeling, water, wastewater, wastewater treatment plants, activated sludge systems

Determining and modeling the dispersion of non point source pollutants in Lake Victoria: A case study of Gaba Landing site in Uganda

Lake Victoria is an important source of livelihood that is threatened by rising pollution. In this study, pollutants in runoff are characterized and their dispersion after they enter the lake is measured and modeled at different points in the study areas. The objective is to develop a one dimensional mathematical model which can be used to predict the nutrient (ammonia, nitrite, nitrate, and phosphate) dispersion distances within the lake. A comparison between rain period nutrient concentrations and dry period nutrient concentrations within the Lake showed an elevation in nutrient levels during the rainy season, with exception of nitrate. However, nitrate had high levels during the dry season. Ammonia was found to disperse to horizontal distances of 38 m; nitrite 45 m, with nitrate and phosphate each attaining distances of 34 m. Measured nutrient concentration within the Lake compared well with model estimations.Key words: Modeling, water quality, nutrients, lake Victoria

Options for wastewater management in Harare, Zimbabwe

The capital city of Zimbabwe, has adopted an urban water cycle that is geared towards high level service provision. Water supply and sewerage/sanitation coverage amounts to over 98%, which makes Harare with the highest coverage. The city's high volume of water abstraction from its main water resource, Chivero, however, can no longer be sustained. The lake has been seriously polluted by large volumes of (partially) treated effluents from wastewater treatment plants in Harare and the neighbouring town of Chitungwiza. It also receives pollution from agricultural, solid waste, industrial, and natural sources. Most of the wastewater treatment plants in the lake's catchment are overloaded and they experience frequent breakdowns. This situation has been worsened by repeated years of drought, resulting in the accumulation of nitrogen and phosphorous in the lake. The negative impacts of this have been reflected in periodic fish kills, proliferation of algae and water hyacinth, and the reduction in biological diversity. Other related problems are difficulties in potable water treatment and clogging of irrigation pipes.There is now an urgent need to control pollution loads and to remove contaminants that haveaccumulatedin LakeChivero over many years. A great deal could be achieved through rational management of the urban water system and the associated nutrient cycle. This should be based on an integrated approach that includes reduction of water consumption, and the wise use of water through pollution prevention/reduction measures. On the water supply side, available options include reduction of water losses (now at ± 30%), water-saving installations (in households, commerce, and industry), direct reuse ( e.g ., greywater), and alternative water resources ( e.g. , rainwater harvesting and groundwater). On the wastewater side, options available include onsite, decentralised and centralised treatment plus reuse.The general objectives of this research were to assess the contribution of wastewater from Harare to the nitrogen and phosphorous inflows into Lake Chivero and, based on this assessment, to formulate feasible sanitary engineering solutions to the problem of excessive nutrient inflows into the lake. The research specifically targeted nutrients because these are the major problem parameters. BOD is largely taken care of via current wastewater treatment and river self-purification processes. The general strategy was to intervene at various levels; i.e. , property, decentralised and centralised levels, with various options aimed at reducing water use and limiting wastewater production and reusing or recycling water and nutrients. This strategy would reduce nitrogen and phosphorous flows to the lake, whilst increasing water availability.An extensive water quality and quantity monitoring study in the Chivero catchment was carried out from June 2000 to December 2001 to assess the current situation in terms of water use, treatment and reuse levels, and flow balances. In addition, current contributions of wastewater discharges to nutrient flows in the rivers and Chivero were assessed. Intervention strategies were developed based on an approach, referred to as the " 3-Step Strategic Approach " to wastewater management. The steps include: 1) pollution prevention/reduction at source, 2) treatment in the direction of reuse, and 3) disposal with stimulation of self-purification capacity of the receiving water body. The three steps should be considered in this chronological order. Options considered include source control by the users (residents, industries, etc) using various strategies such as greywater separation and reuse, implementation of toilets with urine separation, and other ecological methods of wastewater management. Other possible options are invoking better behaviour through fees and information, and user responsibility through education, legislative changes and stricter controls over industry. Options for boosting the self-purification capacity of water bodies include introducing wetlands into the river system via natural overflow, land irrigation, reducing retention time in the lake, etc. Flexible and differential solutions were developed for each landuse category (commercial, industrial and residential).The results of this study confirmed that wastewater plays a major role in the pollution of Chivero. Wastewater contributed over 50% of the annual water flows in the major inflow rivers of Marimba and Mukuvisi. Water quality was found to be an urgent problem that requires immediate action whilst water scarcity was considered a medium-term problem. The river water quality for points upstream and downstream of wastewater discharge points were far above the 0.03 mg/l TP required for avoiding excessive plant growth in rivers. The lake nutrient concentrations were 2.01.3 mg/l TN and 0.6 ±3 mg/l TP ( standard deviation), reflecting a hypertrophic status. Nearly 70% of the annual phosphorus inflows were retained within the lake, which had a hydraulic retention time of 1.6 ±1.1 years based on rainfall years 1981/2 to 2000/1. However, for the monitoring period, the hydraulic retention time reduced to 0.4 years due to the heavy rains received in that period. Besides the need to substantially reduce nutrient inflows in Lake Chivero, adequate water inflows are also essential for the flushing out of nutrients from the lake, especially phosphorus. The continued accumulation of phosphorus in the lake sediments leads to an internal phosphorus cycle, further complicating remedial measures. The effective reduction of nutrient inflows into Lake Chivero hinges on solutions related to wastewater management. It is in this context that the "3-Step Strategic Approach" was applied, focusing on wastewater treatment and reuse options at onsite, decentralised, and centralised levels. An aggregation of these options led to the development of short-term, medium-term, and long-term solutions. It was estimated that significant improvements in the lake water quality (to about 0.4 mg/l TN and 0.07 mg/l TP in the medium-term) could be achieved by applying the measures suggested in this dissertation. In addition, the treatment of part of the effluent to tertiary standard and subsequent discharge into Lake Chivero could also reduce the lake hydraulic retention time to below 0.5 years, thereby enhancing the flushing out of nutrients. It was concluded that both water quality and quantity problems in the Chivero catchment could be significantly reduced via improvements in wastewater management in combination with the control of other point and non-point sources of pollution

Estimating runoff from ungauged catchments for reservoir water balance in the Lower Middle Zambezi Basin

The Lower Middle Zambezi Basin is sandwiched between three hydropower dams; Kariba, Kafue (Itezhi-tezhi) and Cahora Bassa. The operation of the upstream dams impacts on the inflows into the downstream Cahora Bassa Dam which, in turn, affects the area inundated upstream of the Cahora Bassa Dam. This study applied a rainfall-runoff model (HEC-HMS) and GIS techniques to estimate both the gauged and ungauged runoff contribution to the water balance of Cahora Bassa. The rivers considered in the study are the Zambezi, Kafue, Luangwa, Chongwe, Musengezi and Manyame. Missing data were generated using the mean value infilling method. The DEM hydro-processing technique was used to determine the spatial extent of the ungauged area. A hydrological model, HEC- HMS, was used to simulate runoff from the ungauged catchments. Results from the study show that the ungauged catchment contributes about 12% of the total estimated inflows into the Cahora Bassa Dam. Averaged results over 30 years show total inflows of 71.73 x 109 m3/yr, total outflows of 52.25 x 109 m3/ yr and a residual storage of 20 x 109 m3/yr. The study successfully estimated the water balance of the Middle Zambezi Basin which, in turn, may help to inform the operation of the Cahora Bassa Dam and management of artificial floods in the basin.Keywords: Cahora Bassa, DEM hydroprocessing, HEC-HMS, remote sensing, reservoir operation, runoff simulatio

Sewage discharges and nutrient levels in Marimba River, Zimbabwe

Population distribution, land-use, industrial activity, urban agricultural activities, and pollution control strategies in Harare, Zimbabwe, have all changed significantly since independence in 1980. The effects of these changes on water quality and the consequences on human and ecological health are of major economic and social importance. This study looked at pollution in the Marimba River, one of the major inflow rivers into the Lake Chivero, Harare city\'s main water supply source. The river was characterised using standard physical and chemical methods to assess water quality from June 2000 to December 2001. Monthly sampling results indicated high pollution levels, especially nutrients. Nitrogen values averaged 13.5 + 2.0 mg.l-1 as N, and phosphorus 2.6 + 0.6 mg. l-1 as P at the river\'s discharge point into Lake Chivero. The high nutrient values, exceeding acceptable limits, are indicative of industrial, agricultural, and sewage discharges upstream. The gross river pollution levels are detrimental to the downstream Lake Chivero. It was concluded that although there are other significant incidental sources of nutrient pollution in Marimba River, sewage effluent is by far the major polluter. It was established that pond overflow from Crowborough Sewage Treatment Works is the major source of sewage inflows and treating this to tertiary standard would result in 46% and 35% reductions respectively for total nitrogen and total phosphorus loads in the Marimba River. Water SA Vol.30(1): 107-11

Heavy metal pollution and nutrient levels in Lake Muhazi, Rwanda

Most tropical African lakes are facing problems of rapid population growth in the riparian communities, which normally discharges pollution loads into the lakes. This has led to the deterioration of water quality in receiving lakes. Some lakes are getting eutrophic whilst others are facing problems of siltation and heavy metal discharges, thereby reducing their economic and aesthetic values. Some lakes are experiencing a decrease in fish production. In Rwanda, generally the surface waters are full of sediments caused by soil erosion. The proliferation of water hyacinth and other aquatic weeds is now a common phenomenon. The reduction in lake water levels and low fish production are also main problems of lakes in Rwanda. In particular, Lake Muhazi has experienced a dramatic decrease in fish production since the eighties, typified by low water transparencies and high turbidities. The lakeshores are now being developed for ecotourism and this requires sound environmental management to make the planned activities viable. The purpose of this study was to investigate the nutrient status of Lake Muhazi by assessing vertical and horizontal distributions of nutrients in the Lake. The parameters studied are nitrogen, phosphorous, chlorophyll a, and Secchi disc measurements. The nutrient levels were determined over a period of four months from July 2007 to October 2007. Four sampling stations were located within the lake and the sampling was done fortnightly. The samples were collected at depths of 0.5 m, 2 m, 5 m, and 1 m from the bottom of the lake. Samples were taken using a Van dorn Bottle water sampler and were analyzed for heavy metals, TP, TN, NO2-N, NO3-N and chlorophyll-a using the Standard Methods. Field measurements of temperature, transparency (Secchi disc), and pH were also measured. The results indicate that the nutrient levels in the Lake are higher than previously reported figures and urgent intervention is required. It was observed that the high nutrient levels in the lake are related to landuse activities in the catchment. It is recommended that farming practices and erosion be controlled on the catchment to contain pollutant discharges into the Lake

Heavy metal pollution and nutrient levels in Lake Muhazi, Rwanda

Most tropical African lakes are facing problems of rapid population growth in the riparian communities, which normally discharges pollution loads into the lakes. This has led to the deterioration of water quality in receiving lakes. Some lakes are getting eutrophic whilst others are facing problems of siltation and heavy metal discharges, thereby reducing their economic and aesthetic values. Some lakes are experiencing a decrease in fish production. In Rwanda, generally the surface waters are full of sediments caused by soil erosion. The proliferation of water hyacinth and other aquatic weeds is now a common phenomenon. The reduction in lake water levels and low fish production are also main problems of lakes in Rwanda. In particular, Lake Muhazi has experienced a dramatic decrease in fish production since the eighties, typified by low water transparencies and high turbidities. The lakeshores are now being developed for ecotourism and this requires sound environmental management to make the planned activities viable. The purpose of this study was to investigate the nutrient status of Lake Muhazi by assessing vertical and horizontal distributions of nutrients in the Lake. The parameters studied are nitrogen, phosphorous, chlorophyll a, and Secchi disc measurements. The nutrient levels were determined over a period of four months from July 2007 to October 2007. Four sampling stations were located within the lake and the sampling was done fortnightly. The samples were collected at depths of 0.5 m, 2 m, 5 m, and 1 m from the bottom of the lake. Samples were taken using a Van dorn Bottle water sampler and were analyzed for heavy metals, TP, TN, NO2-N, NO3-N and chlorophyll-a using the Standard Methods. Field measurements of temperature, transparency (Secchi disc), and pH were also measured. The results indicate that the nutrient levels in the Lake are higher than previously reported figures and urgent intervention is required. It was observed that the high nutrient levels in the lake are related to landuse activities in the catchment. It is recommended that farming practices and erosion be controlled on the catchment to contain pollutant discharges into the Lake