Greywater reuse: A strategy for water demand management in Harare?

Abstract

Greywater is wastewater from baths, sinks and washing machines, accounting for about 60% of the outflow from homes. It contains little pathogens and 90% less nitrogen than toilet water, so does not require the same treatment process. With the increasing demand for freshwater, its use may reduce irrigation water needs, increasing its availability of freshwater for other primary uses. Agriculture is the main water consumer in Africa, which cannot be compromised due to its role in domestic food security and export supplies. The purpose of this paper is to demonstrate findings of the research done on benefits of greywater reuse in some countries, applicable to African countries. In Australia, greywater reuse has reduced freshwater demand, strain on wastewater treatment plants and energy consumption. Aquifer recharge has improved due to increased infiltration flows from greywater uses. In Lebanon, greywater is a valuable resource for encouraging plant growth from nutrients that may otherwise have been wasted. Palestine shares similar climate and water scarcity conditions with most arid sub-Saharan African countries, yet utilizes grey water in production of crops and citrus fruits. Thus use of grey water should be possible in African cities such as Harare, where nearly two thirds of the population rely on agriculture for livelihoods. The problem of blue green algae in sewerage ponds and water reservoirs is significantly reduced by household reuse of grey water in Mexico. Water savings are increased and expenses reduced, as illustrated by the reduction in consumption of municipality freshwater supplies in South African urban areas. Rural communities and schools in Namibia and Egypt have raised funds from grey water reuse in banana plantations. A possible constraint to this strategy could be the unavailability of appropriate technology for primary treatment of grey water before reuse. This strategy may pose health risks where water quality tests are unknown or unavailable but can be overcome through educational campaigns. Supporting data and evidence on safety and benefits of grey water is required to achieve regulatory approval of grey water use.

Introduction

The quality of raw water in the source of drinking water for over two million Zimbabwean people, Lake Chivero in Harare has been decreasing over the years (Moyo, 1997). This was mainly due to non-point sources of pollution and subsequent discharge of large volumes of partially treated effluents domestic, sewerage and industrial wastes from the city’s waste treatment plants and the neighbouring town of Chitungwiza (Nhapi, 2004). The frequently breaking down and overloaded plants are inefficient and so release effluent discharged in a state that the natural environment cannot bear to self purify. This has since increased the nutrient enrichment and subsequent overgrowth of algae (Microcystis aeruginosa) the aquatic weed (Eichhornia crassipes), covering about 40% of the Lake Chivero surface area (Magadza, 2003). Water quality and quantity have since been affected too, and the entire ecosystem of fish and other biota.

Secondary effects of water hyacinth invasion in Lake Chivero included increased cost for water treatment in Harare due to the decline of the quality of the water. The dosages of alum, for example rose from 40 mg l⁻¹ in 1982 to 100 mg l⁻¹ in 1995 (Jaravaza, 1997), and continues to increase. Other problems associated with deterioration of the water are poor sedimentation, clogging of filters, disorders in colour, smell and taste, increasing pH, alkalinity and low turbidity which make the water treatment process more difficult than when treating less polluted waters. As raw water became poorer and poorer, more chemicals were adopted to get the sufficient quality of drinking water the various water users could use. This also increases the cost of water to the residents.

Water supplies to the Zimbabwean capital, Harare, have seen a steady decline trend in the past years (Gumbo, 1997, The Herald, 2006). This have since tightened water rationing, making some residential areas to go without the basic necessity for an average of two days or more per week. The operation has also affected production outputs in the suburbs and industries. Drinking water from the Morton Jaffray and Prince Edward Water Treatment Plants in the country’s capital also supply smaller surrounding towns such as Chitungwiza, Epworth, Norton, Ruwa among others. Water shortages have led to new innovations, due to decreasing quality and quantity, and increasing population in the towns. Norton Town Council which demands about 20 Ml per day; is spending $4 billion on the construction of its own water treatment plant that will allow the town of more than 60 000 people to stop depending on Harare for supplies (The Herald, 2006). Ruwa Local Board has almost completed its own water treatment plant and is augmenting imported Harare water with its own, thereby lessening the biting water shortages. Greywater reuse is another water demand option, which may reduce water consumption, and lower the cost on the rate payer.

Water demand management has been recognized as an effective way of managing the scarce water resources (Stiles, 1997). Water pricing has been adopted, where water is treated as an economic resource but the marginal poor still cannot get the amounts they need, since they cannot afford to pay for it. Recycling or rather grey water re-use may be a possible strategy for saving water in Harare. With the increasing demand for freshwater and increasing water scarcity indices in Sub-Saharan Africa (Table 1, Yilma et al., 1998); this strategy may reduce water needs, increasing its availability for primary uses, such as domestic and animal life. Succinct use of the quantities available is an option to consider, even in reduction of droughts effects (Hirji et al., 2005). Grey water reuse is an important environmental and economic initiative. Food security is one major Millennium Development Goal (MDG), through the reduction of hunger and poverty, which can be achieved through greywater irrigation.

While agriculture is the main water consumer in Africa, it cannot be compromised due to its role in domestic food security and export supplies (Chiuta et al., 2002). However, it is the largest consumer of freshwater resources, currently accounting for about 70% of global water diversions (Raschid, 2004). With increasing demand from municipal and industrial sectors, competition for water will increase and it is expected that water now used for agriculture will be diverted to the urban and industrial sectors. One response to this squeeze on agricultural water supply is to promote greater use of treated urban wastewater for irrigation (Gumbo, 1997). At household level, the re-diversion of this treated wastewater may be expensive, and the option would be succinct reuse of grey water. For example, lawns are often over-watered and over-fertilised. Lawns consume some 40% of Harare’s high quality water supply.

Grey water, or sullage, consists of all non-toilet household wastewater from showers, baths, hand basins, washing machines, and kitchen sinks. This study aimed at investigating greywater reuse as an option for water demand management in urban and rural settings, using Harare as a case study. Benefits and adoptability of the mechanisms were investigated, based on the use of greywater in other countries. A scenario for the future is presented where greywater reuse is not only permitted but in many situations is compulsory.