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2.1 Characteristics of wastewater
Wastewater comprises of many features that differentiates it from the naturally occurring water body. According to FAO (1992), municipal water is mainly comprised of 99.9% water together with relatively small concentrations of suspended, dissolved organic and inorganic solids that include carbohydrates, lignin, fats, soaps, detergents, proteins, natural and synthetic organic chemicals from industries. Wastewater may contain all kinds of chemical and biological pollutants that include heavy metals, nitrogen, phosphorus, detergents, pesticides, hydrocarbons, viruses, bacteria, and protozoa. Some heavy metals are micronutrients and required in trace amounts by living organisms for their normal metabolic function (Gad: 2016).
2.1.1 Nutrients
Nutrients are among the key parameters that define the water quality in surface and underground waters and nutrient removal from wastewater is important before effluent is discharged into receiving water bodies or reused in agriculture or aquaculture (Mayo: 2005). However, increased nutrient loading can lead to eutrophication (Gücker et al: 2006) and temporary oxygen deficits (Rueda et al: 2002). The net effect of eutrophication on an ecosystem is usually an increase in the abundance of a few plant types to the point where they become the dominant species in the ecosystem and a decline in the number and variety of other plant and animal species in the system (Bernard: 2010). The effluent from anaerobic ponds usually has higher concentrations of ammonia than in raw sewage and in facultative and maturation ponds, ammonia is incorporated into algal biomass (Kayombo: 2015). In conditions of high photosynthetic activity, the pH can rise to values higher than 9.0, providing conditions for the stripping of the NH3 and the high algal production contributes to the direct consumption of NH3 by the algae (Sperling 2007).
2.1.2 pH
Values of pH in ponds wastewater are important for removing heavy metals that may be present. At acidic pH, heavy metals tend to exist as free metal ions while around neutral at around 6–9 pH some precipitate as hydroxides or other insoluble species if the appropriate co-ion is available (Mara: 2003). In facultative and maturation ponds this rise in pH can be related to the rapid photosynthesis of algae, which consumes carbon dioxide faster than it can be replaced by bacterial respiration. Thus as a result carbonate and bicarbonate ions dissociate. Algae fix the resulting carbon dioxide while hydroxyl ions accumulate so raising pH (Gad: 2016).
2.1.3 Organic matter
Wastewater contains organic matter which comes from organic products such as vegetables and the organic matter is found throughout the pond system. Shon (2005) highlighted that the presence of trace organic pollutants in wastewater has been the cause of increasing public concern in recent decades due to potential health risks. Thus the facultative ponds are designed for Biological Oxygen Demand removal based on their surface organic loading which is the quantity of organic matter, expressed in kilograms of BOD per day, applied to each hectare of pond surface area (Kayombo: 2015). A relatively low surface organic loading is used to allow for the development of an active algal population (Pena: 2004). Verbyla (2017) coincided by stating that the main function of anaerobic, facultative and aerated ponds is the removal of carbon-containing organic matter. Okoro (2016) found that wastewater from animal origins like piggeries contained higher concentrations of organic content which required further treatment. However, the organic matter content decreases as the influent moves from one stage to the other and maturation ponds have lower organic load as compared to fulcatative ponds. The algal populations are much more diverse than that in facultative ponds and Pena: (2004) concurs that algal diversity increases from pond to pond along the series.

2.1.4 Heavy metals
The persistence of heavy metals in wastewater is due to their non-biodegradable and toxicity nature (Jern: 2006). Some of the negative impacts of heavy metals on plants include decrease of seed germination and lipid content by cadmium, decreased enzyme activity and plant growth by chromium, the inhibition of photosynthesis by copper and mercury, the reduction of seed germination by nickel and the reduction of chlorophyll production and plant growth by lead (Torresdey: 2005). The impacts on animals include reduced growth and development, cancer, organ damage, nervous system damage and in extreme cases, death (Canada Gazette: 2010). The clinical signs of zinc toxicosis include diarrhoea, vomiting, icterus (yellow mucus membrane), bloody urine, anaemia, kidney failure and liver failure (Duruibe: 2007). Also, lead toxicity can have many effects depending on age of the person which include irritability, hyperactivity, anaemia whilst acute toxicity can result in delirium, encephalopathy, anorexia and in some cases, severe diarrhoea and dehydration (Kathuria: 2018).

2.1.5 Microorganisms
Microorganisms assist algae in the breakdown and settlement of degradable organic matter, generally before discharge of treated effluent to land (Australia department of water: 2009). Although most organisms in biological wastewater treatment plants are microscopic in size, there are some organisms such as bristle worms and insect larvae that are macroscopic in size (Geradi: 2006). Potential pathogens in wastewater effluents include various genera of bacteria, viruses, protozoa, and helminthic ova, whose presence in output wastewater can negatively affect receiving environments (Australia Department of Water: 2009) and also, human health (Liu: 2017). Shon (2005) concurred by stating that the microbiological composition of domestic wastewater often contains coliform organisms, faecal streptococci, protozoan cysts, and virus particles. These constituents make the wastewater a health risk and this was noted by Mutengu (2006) who said that wastewater is likely to contain pathogenic organisms similar to those in the original human excreta thereby making the wastewater dangerous.

2.2 Waste stabilization ponds
Waste stabilization ponds are man-made water bodies with the function of accepting, storing and processing waste water so that it becomes environmentally friendly before it is released to the environment. Waste stabilisation ponds are designed to treat waste water using natural means and this was echoed by Verbyla (2017) who defined waste stabilization ponds as sanitation technologies that consist of open basins that use natural processes to treat domestic wastewater, septage, and sludge, as well as animal or industrial wastes. Phuntsho (2009) also described waste stabilization ponds as systems that consist of a series of anaerobic, facultative and maturation ponds or several series that lie in parallel. There are three types of waste stabilization ponds in common use namely anaerobic, facultative and maturation ponds. Due to their long hydraulic retention times, the ponds are more resilient to both organic and hydraulic shock loads than other wastewater treatment processes (Gad: 2016).
Waste stabilization pond system is considered as the most appropriate system to treat the increasing flows of urban wastewater in tropical and subtropical regions of the world (Jeroen: 2007). This notion was also supported by Mahmood (2013) who highlighted that a total of 1 304 stabilization ponds were currently being used as the principal method of method of sewage treatment serving a population of 2 146 951 in the United states. This indicates the usefulness of the pond system in treating wastewater.
2.2.1 Inputs of waste stabilization ponds
Waste water is introduced into the waste stabilisation ponds through the inlet channels which are connected to the ponds. The influent wastewater enters at one end of the pond, stays for several days whilst activities of purification would be taking place and leaves at the opposite end (Sperling: 2007). The influent normally consists of blackwater, grey water, brown water dissolved matter, insoluble matter, suspended material, organic material, faeces and excreta. Beyene (2011) hinted that waste stabilization ponds may also receive untreated wastewater that has gone through preliminary treatment processes like screening and grit removal or they may receive secondary effluent from some other treatment process, such as anaerobic reactors, activated sludge, or trickling filters.
Figure 3
Inputs and outputs of waste stabilisation ponds

2.2.2 Outputs of waste stabilisation ponds
The outputs from waste stabilization systems include the treated effluent that is normally released into the environment. The effluent also includes sludge, fertigation and biogas (Verbyla: 2017). There is also sludge that is produced by the ponds and according to Power and Water Corporation (2011) report, sludge may contain pathogens and therefore a sludge disposal area must be lined to ensure that no leachate must enter the local aquifers.
2.3 Global and Local Trends in waste ponds usage
Waste ponds have been used the world over the past 50 years for municipal and industrial waste water. The waste water treatment has been accepted and used to change the physical, chemical or biological characteristics of the waste (Quiroga: 2002). This can be supported by the fact that currently, there are more than 2 500 waste stabilisation pond systems in France and around 3 000 in Germany including around 1 500 in Bavaria alone and also 7 000 in the USA (Mara: 2008). However, waste stabilization ponds are also used in other industrialized and developing countries but not in such large numbers. Even though used in less numbers, waste stabilization ponds are the preferred wastewater treatment process in developing countries where land is often available at reasonably low cost and skilled labour is in short supply (Gratziou: 2012). There is also abundant sunlight in developing countries like Africa which is beneficial to the processes that occur in the ponds. According to Arthur (1983) the problems associated with the disposal of domestic and other liquid wastes have grown with the world’s population and the problems are particularly acute in developing countries where only 32% of the population have adequate excreta and sewage disposal services and the situation is worsening. This is despite the fact that waste stabilisation ponds can be used in centralized or semi-centralized sewerage systems, serving cities or towns and they can also be used as onsite systems serving a single entity such as highway rest area or a community centre (Verblya : 2017). Also, the domestic and liquid waste disposal problem contradicts Weaver (2012) who said that wastewater treatment is a requirement worldwide to protect both public health and the environment from anthropogenic activities. Roughly 10 % of the world’s wastewater is currently being used for irrigation and in developing countries especially China and India, an estimated 80% of wastewater is used for irrigation (Cooper: 1991). Thus the waste water quality should be closely monitored to determine whether it is well treated and this was supported by Pena (2004) who highlighted that the quality of the final effluent should be regularly determined at all waste stabilisation pond sites and samples should be analysed for those parameters for which the effluent standards have been set by the local environmental regulator such as BOD, suspended solids, pH, Escherichia coli or faecal coliforms and helminthic eggs if the effluent is to be reused in agriculture.

In Zimbabwe, algae based waste stabilization ponds are used for wastewater treatment in most small urban areas and this is mainly because small urban centres lack the financial resources to put up the modern state of the art treatment systems and that they only produce low volumes of mainly domestic wastewater( Dalu: 2003). Zimbabwe also has four major cities which have a population of more than 1 million people namely Harare, Bulawayo, Mutare and Gweru and according to Mudyiwa (2006), of the 137 wastewater treatment in the country, 101 are waste stabilisation ponds. This means that there are many ponds however, local authorities who are responsible for properly running these waste stabilisation ponds face major financial constraints to overhaul the aging wastewater infrastructure (Thebe: 2012). The small urban centres also have the land on which to construct waste stabilization ponds that have low operation and maintenance costs