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2.1 characteristics of wastewater
wastewater conists of many features that differentiates it from the naturally available water body. according to fao 1992 municipal wastewater is made up of 99.9% water together with low amounts 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 impurities that comprise heavy metals phosphorus nitrogen detergents pesticides hydrocarbons viruses bacteria and protozoa. some heavy metals are micronutrients and required in trace quantities by living organisms for their normal metabolic function gad: 2016

2.1.1 nutrients
nutrients are among the key parameters that define the quality of water on surface and underground water and nutrient removal from wastewater is vital before the effluent is discharged into receiving water environments or reused for 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 efficacy of eutrophication on an ecosystem is usually a boom 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 within the system bernard: 2010 the effluent in anaerobic ponds usually has high amounts of ammonia as compared to raw wastewater and in facultative and maturation ponds ammonia is incorporated to form algal biomass kayombo: 2015 in conditions of high photosynthetic action, the ph can rise to values more than 9.0 supplying conditions for the removal of the nh3 and the high algal production promotes to the direct consumption of nh3 by the algae sperling 2007

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2.1.2 ph
values of ph in wastewater plants are vital 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 types when the appropriate co-ion is available mara: 2003 in facultative and maturation ponds this increase in ph can be attributted 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 materials like vegetables and the organic matter is found accross the whole pond system. shon 2005 hinted on the presence of trace organic pollutants in wastewater which has been the cause of increasing public outcry in recent decades due to potential health risks. thus the facultative ponds are designed for biological oxygen demand removal based on their surface organic burden 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 building of an active algal population pena: 2004 verbyla 2017 coincided by stating that the main uses of anaerobic facultative and aerated ponds is the scrapping of carbon-containing organic matter. okoro 2016 found that wastewater from animal origins like piggeries contained high concentrations of organic matter which required more 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 quantities are much more diverse than those in facultative ponds and pena: 2004 concurs that algal diversity accelerates from pond to pond within 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 decrease 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 harm 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 side 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 various pathogens in wastewater effluents include various types of bacteria viruses, protozoa and helminthic ova whose presence in effluent wastewater can negatively affect receiving environments australia department of water: 2009 and also human health liu: 2017 shon 2005 concurred by saying that the microbiological constituents 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 harmful microorganisms 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 stabilization 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 comprise of open water basins that use natural processes to treat domestic wastewater sewage 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 which are anaerobic facultative and maturation ponds. due to their long mechanical 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 taken into account as the most appropriate system to treat the increasing flows of wastewater coming from urban areas 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 systematic way 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 stabilization 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 obtain raw wastewater that has gone through some pretreatment processes like screening of large solids and grit removal or they may receive partly treated inffluent 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 comprise the treated effluent that is normally released into the environment. the effluent also contains 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 developed countries but not in such large numbers. even though used in less numbers waste stabilization ponds are the most admired 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 demerits associated with the disposal of domestic and other liquid wastes have grown with the worlds population and the problems are particularly serious 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 stabilization ponds can be used in centralized or semi-centralized sewerage systems serving cities or towns and they can also be utilized as onsite systems that cater for 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 worlds 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

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