Published on International Journal of Engineering & Industry
Publication Date: July 10, 2019
Kawareware Casper Takudzwa, Mango Lawrence, Kugedera Andrew Tapiwa & Mango Lovemore
Chimurenga Secondary School, P.O Box 333, Mutoko
Faculty of Agriculture, Department of Agriculture Management, Zimbabwe Open University, 209 Hay Road, Bindura, Zimbabwe
Department of Livestock, Wildlife and Fisheries, Gary Magadzire School of Agriculture and Natural Sciences, Great Zimbabwe University, P. O. Box 1235, Masvingo, Zimbabwe
Magunje High School, P.O Box 30, Murewa, Zimbabwe
Waste stabilization ponds have proven to be the effective alternatives for treating wastewater, and are efficient systems in removing excreted pathogens. This study was carried out to investigate the impacts of waste stabilization pond effluent on the water quality of the receiving Chikondoma stream in Mutoko. Field observations and a detailed water quality analysis were used for data collection. Water samples were collected from points selected upstream of the discharge point, at the discharge point and downstream of the discharge point. This was done to establish changes in the river water quality; before the discharge point, at the discharge point and downstream. Samples were analyzed for pH, temperature, Dissolved Oxygen, Biological Oxygen Demand, nitrates as well as sulphates. The obtained results were compared with acceptable WHO standards. The water quality analysis results revealed that the treatment plant exhibited effluent qualities that are acceptable with WHO standards in some parameters (like pH, temperature and sulphate with average values of 6.97, 25.34oC and 2.47 mg/L respectively) at the discharge point. At the discharge point some parameters fell short of standard requirements (mean values of BOD, DO and nitrate were 34.18 mg/L, 3.70 mg/L and 15.84mg/L respectively) that are critical for the provision of clean and safe water. This implies that the effluent presented significant risks of water pollution and environmental damage. The results also indicated that waste stabilization pond effluent is the major source of pollution to Chikondoma stream. It is recommended that there is need to carry out further studies using water quality models that can trace the fate and transport of pollutants downstream.
Keywords: Waste stabilization pond, effluent & water quality.
Waste stabilization ponds (WSPs) are man-made earthen basins, comprising at any one location or more series of anaerobic, facultative and depending on the desired effluent quality and maturation ponds (Mara and Pearson, 1998; Rao, 2005). Sewage can be defined as discharges from domestic and sanitary appliances or a complex mixture of soluble and insoluble wastes from various sources (Muchabaya and Mwanunzi, 2006). According to Meybeck and Helmer (1996), water quality is a term used to express the suitability of water to sustain various uses or processes. Waste stabilization ponds are particularly suited to tropical and subtropical countries since sunlight and ambient temperature are key factors in their performance.
The principle objective of waste water treatment is generally to allow domestic and industrial effluents to be disposed without posing danger to human health or the environment. Waste water treatment ponds are a way for water recycling and indeed an effective technique for waste water treatment (Mara, 2004). Waste stabilization ponds are generally considered the treatment technology of choice for municipal waste water in many parts of the world because of their effectiveness in pathogen removal and their low operation and maintenance costs (Muchabaya and Mwanunzi, 2006). However, there are several technical and environmental factors which determine their effectiveness. Such factors are among others, the configuration and size of the ponds with respect to the sewage discharges, ambient temperature and their maintenance.
The treatment plant in Mutoko mainly treats sewage which originates from domestic wastes. Domestic wastes are mostly organic solids, dissolved or otherwise, from toilets and laundry detergents. The sewage also contains organic plant nutrients such as phosphates, potash and nitrates from detergents. The ponds treat sewage from the old locations in Mutoko Township. The new locations in the area use septic tanks and soak-aways as a way of sewage disposal. According to UNICEF, (2010) sewer system in the Central Business District and old Chinzanga are overloaded and constantly block and spill onto the roads. The treatment ponds are full of sludge, no longer work well and act as holding ponds for the incoming waste. There is evidence of poor maintenance and operation of waste water treatment ponds from the overgrowth of aquatic weeds which cover the surface of the ponds and produce foul smell. According to Rao (2005), the decay of such plants and their decomposition can deplete dissolved oxygen of the water to levels below the critical levels needed to sustain life of aquatic organisms.
The fence protecting the area has been removed exposing the treatment ponds to livestock which disturbs their functionality. The area is being used for dumping solid waste by nearby residents. Chikondoma stream is a tributary to Mudzi River which feeds into Mother of Peace dam. The stream flows through locations such as Chikondoma Township and some villages, before discharging into Mudzi River approximately 6 kilometers from the effluent discharge point. Mother of Peace dam is mainly used for irrigation water supply to the Mother of Peace farm and other nearby farms. This study sought to evaluate ponds performance, effluent quality and estimate their contribution in polluting Chikondoma stream.
2. Materials and Methods
2.1 Description of the study Area
The study was conducted in Mutoko which is a rural district town in Mashonaland East province of Zimbabwe. It is 140 km north east of Harare and can be accessed through the highway to Nyamapanda Border Post. It lies at latitude 17° 15′ S and longitude 32° 15′ E at an elevation of 1250m above sea level. It is situated in agricultural region 3 which on average receives an annual rainfall of between 500mm and 1000mm. The main business activity is agriculture; horticulture including fresh vegetables (peas, baby corn, tomatoes, onions, cucumbers, butternut, leaf vegetables, sweet corn among others), fruits (mangoes, guavas) and crop husbandry (cotton, maize, millet and rapoko) and various industries including mining of black granite and tantalite. UNICEF (2010), reported an estimated population of 11,380 in 2009, and there are 4400 residential stands of which about 50% are fully established and the rest are under development.
2.2 Sanitation Systems in Mutoko
There are two different systems of waste water treatment in Mutoko area. New locations are using septic tanks while old locations are connected to the waste stabilization ponds. The sewage treatment plant consists of one anaerobic, one facultative and one maturation pond connected in series. Influent to the ponds is mainly domestic sewage. Effluent from the waste stabilization ponds is discharged into Chikondoma stream. Water from the stream is used for domestic purposes by nearby communities (Old Chinzanga, Chinzanga Extension 1, Chinzanga Extension 2 and Chikondoma high density location). Sources of pollution along Chikondoma stream include sewage leakages, waste dumping and runoff from garages, households and business areas.
2.3 Sampling and Sample Analysis
2.3.1 Sampling Point Selection
Three sampling points were selected along Chikondoma stream. Sampling point 1 (SP1), is approximately 50m upstream of the discharge point. This was considered so as to determine the water quality that is independent from WSP effluent quality (reference point). Sampling point 2 (SP2), at the discharge point, was selected in order to determine WSP effluent quality. Sampling point 3 (SP3), at Chikondoma Bridge, approximately 500m downstream of the discharge point. This was considered so as to determine the effect of the WSP effluent to the water quality since the water is used for domestic purposes by the local residents.
2.3.2 Sampling and analysis
Field sampling was conducted for five weeks from 25 April to 30 May 2018. Surface water samples were collected at each sampling point (3 replicates) in 500ml polythene bottles for laboratory analysis. The bottle was closed soon after filling to the brim. BOD was sampled using dark BOD bottles. The pH and temperature of water were measured on site. Other samples were preserved at 4°C by refrigeration before they were analysed. The samples were analyzed for BOD5, COD, PO42-, SO42- and pH using standard analytical methods as described by Clesceri et al, 1989; EPA, 1992). Table 1 below summarizes the analytical methods.
Table 1: Standard Methods for Chemical Analysis
Parameter Method of Analysis
pH Measured on site using H1255 combined pH meter. The pH meter was calibrated on the instrument and the probe was then inserted into the water sample. The appropriate pH reading was taken (Clesceri et al, 1989).
Temperature (oC) Prior to sample preservation, the temperature was measured on site by placing the probe of the portable thermometer into the collected sample.
BOD5 (mg/l) Dilution method and dissolved oxygen measurement using an unseeded sample was used for BOD measurement. 3 drops of microorganism seed were added to each sample being tested. The seed is generated by diluting activated sludge with deionized water. The BOD test was carried out by fixed aliquot of seed, measuring the dissolved oxygen and sealing the sample to prevent further oxygen dissolving in. The sample was then kept at 20°C in the dark for 5days to prevent photosynthesis and the DO was measured again. The difference between initial and final DO is BOD (Clesceri et al, 1989).
DO (mg/l) Winkler Method.
A 300ml dark BOD bottle was filled with sample water and 2ml of manganese sulphate were immediately added to the collection bottle. 2ml of alkali potassium iodide solution were added to the sample water, the bottle tightly closed with a stopper and the sample mixed by inverting the bottle for several times until a brownish-orange precipitate (floc) was formed. 2ml of concentrated sulphuric acid were added to the sample water using a pipette and the bottle was inverted for several times to dissolve the floc. Then sample was then kept for 8 hours in a cool dark place.
20ml of sample water were isolated into a glass flask and titrated with sodium thiosulphate to a pale straw colour. 2ml of starch solution were added and a blue colour was formed. Titration with sodium thiosulphate was continued until the sample was colourless. The concentration of dissolved oxygen (mg/l) in the sample was equated to the number of millilitres of sodium thiosulphate added.
PO4-P (mg/l) The Vanadomolybdophosphoric Acid Method
The test vial was filled with 10 ml of the sample water. The test vial was inserted into a colorimeter and scanned as a blank. Two ml of VM Phosphate Reagent was added and mixed well. After waiting for a period of 5 minutes, a reading was obtained using the colorimeter.
No3-N (mg/l) Kjedjal Method
100ml of sample was placed in a 200ml long necked flask and 3g of anhydrous sodium sulphate, 0.3g of nitrogen-free mercury oxide and 3 drops of nitrogen free sulphuric acid were added. The mixture was heated over a small flame until it was colourless. It was boiled gently for a further 2 hours
The heated mixture was cooled to room temperature and diluted to 75-85ml with distilled water. A piece of granulated zinc, a solution of 15g of sodium hydroxide and 2g of sodium thiosulphate in 25ml of distilled water were added while shaking the flask and immediately the flask was connected to a distillation apparatus. The liberated ammonia was distilled into 20ml solution of boric acid.
The distillate was then titrated with N/10 sulphuric acid using methyl red as indicator. The operation was repeated using a blank.
Ammonia liberated sample = titre sample – titre blank
Each ml of N/10 sulphuric acid is equivalent to 0.001401g of nitrogen.
SO42-(mg/l) EDTA Titration
100ml of sample water were treated with excess barium chloride to precipitate sulphate ions as barium sulphate. The resulting solution (containing unprecipitated barium ions) was then titrated with EDTA.
Source: (Clesceri et al, 1989; EPA, 1992)
2.4 Data Analysis
The data was analysed using the statistical package SPSS (version 21) to determine significant difference in the levels of the selected chemical parameters (pH, BOD, PO42-, SO42-, No3-N, Temperature, DO) at the upstream, downstream and at the discharge point of effluent.
3. Results and Discussion
3.1 Chemical Analysis Results
The mean water quality characteristics in Chikondoma stream at three sampling points are summarized in Table 2. The results indicate that all the parameters were lowest at the reference site (SP1) and highest at the effluent discharge point (SP2) except for pH and dissolved oxygen. Dissolved oxygen and pH were highest at sampling point 1 and lowest at the effluent discharge point. The pH and sulphate values at all the sampling points were within the range of the recommended WHO standards. Statistical analysis (one way analysis of variance) indicated a significant difference (p<0.05) for DO (p=0.000), nitrates (p=0.000), sulphates (0.005) and BOD5 (0.036) between the sampling points. Table 2: Chemical Analysis Results for Three Sampling Sites 3.2 pH The mean pH values for all the sampling points were found to be within both the WHO standards for drinking water of 7.0 to 8.5 and that for full contact recreation of 6.0 to 9.0 (WHO, 1996). The difference in pH values between the reference point and downstream the discharge point can be attributed to the discharge from the waste stabilization ponds. The pH value at SP3 can be attributed to the dilution effect of the effluent and the mainstream. The trends in the pH values can also be attributed to sulphate concentration which is highest at the discharge point. The drop in pH values may be an indication of the presence of some natural compounds in the effluent such as humic and fluvic acids (Chapman, 1998). 3.3 Temperature The temperature ranges were generally the same at all the sampling points. The mean temperature values recorded were 24.3°C at the reference site, 25.34°C at the discharge point and 24.47oC downstream of the discharge point. The values were generally in the WHO range of below 25oC except at the discharge point (WHO, 1996). According to Jaji et al, (2007), effluent temperature in this range can pose a threat to the homeostatic balance of the receiving water bodies. 3.4 Dissolved Oxygen (DO) The mean DO values obtained were 6.20 mg/L at the reference point, 3.70 mg/L at the discharge point and 4.35 mg/L downstream of the discharge point (SP3). The DO content at the discharge point was lower than from the reference point. This could be attributed to the presence of degradable organic matter which results in a tendency of becoming more oxygen demanding (Obire et al, 2003). Dissolved oxygen concentrations in unpolluted water normally range between 8 and 10 mg/L and concentrations below 5 mg/L adversely affect aquatic life (Rao, 2005). The DO standard for drinking purpose is 6 mg/L whereas for sustaining fish and aquatic life is between 4 to 5 mg/L (EPA, 1992). The DO values obtained from this study at the discharge point and downstream fell short of the WHO recommended standard of 6mg/L (WHO, 1996). 3.5 Biological Oxygen Demand The mean obtained BOD values were 13.47 mg/L at the reference point, 34.18 mg/L at the effluent discharge point (SP2) and 23.31mg/L downstream of the discharge point (SP3). The low values of dissolved oxygen and high values of BOD at discharge point and downstream can be attributed to high competition for dissolved oxygen by the suspended, dissolved substances and micro organisms in the discharged effluent, which is an indication of pollution (Obire et al, 2003). The BOD value at the effluent discharge point was also above the recommended WHO limit of 30 mg/L (WHO, 1996). However, dilution effect was sufficient to lower the BOD levels to below the WHO recommended limit at the downstream of the discharge point. 3.6 Nitrates as Nitrogen The highest nitrite levels were found at the effluent discharge point. The mean total nitrate concentrations obtained were 6.04 mg/L at the reference point, 15.84 mg/L at the discharge point and 12.19 mg/L downstream of the discharge point. The results suggest that effluent largely pollutes the Chikondoma stream as shown by high concentrations of nitrates. The high nitrate concentration at the reference point suggests non point sources of pollution to the stream such as sewage leaks and urban runoff. The decrease in nitrate concentration between the discharge point and downstream can be due to the dilution effect. The total nitrite levels obtained during the study at the discharge point and downstream of the discharge point exceeded the WHO regulatory limits of 10 mg/L. Nitrite is thus considered to pose health problems to communities when the receiving water is used for domestic purposes (WHO, 1996). This may give rise to methaemoglobinemia in infants (Barnes and Bliss, 1983). The nitrite from the treated effluents could be a source of eutrophication for the receiving water. The values obtained from the wastewater treatment plant exceeded the recommended limits of 0 to 0.5 mg/L nitrate (Barnes and Bliss, 1983). 3.7 Sulphates The concentration of sulphates was lowest at the reference point and highest at the discharge point. Although sulphate concentration was highest at the discharge point, it was below the WHO limit of 250 mg/L for the discharge of wastewater into rivers (WHO, 1996).
The water quality analysis revealed that the treatment plant exhibited effluent qualities that meet the acceptable standards in some parameters like pH, temperature and sulphate. It was observed that some parameters fell short of standard requirements that are critical for the provision of clean and safe water (BOD, DO and nitrate). Without any prior treatment of the water, this could pose significant health and environmental problems to communities which rely on the receiving water for domestic use. This may also affect the health status of the aquatic organisms in the receiving water. The water quality analysis results also showed that the effluent from the waste stabilization ponds is a major polluter of Chikondoma stream since all parameter concentrations were highest at the discharge point.
Based on the findings, the study, recommended the use of water quality models that can trace the fate and transport of pollutants once they are discharged into rivers/streams. Application of these models will allow the governing authorities to advise the communities of where they should collect water for domestic use. Further studies on the relationship between pollution levels in Chikondoma stream and the associated rivers should also be carried out. It is also recommended that further studies on water quality over different seasons should be carried out so that water collection points can be defined according to the seasonal calendar. All government departments and agencies concerned with water quality in Mutoko should evolve measures to check and ensure that discharged effluents comply with laid down rules and regulations.