Towards a Sustainable Sanitation System for the City of Windhoek

April 17, 2015, 11:35am

Towards a Sustainable Sanitation System for the City of Windhoek

A recent News Article broadcast by NBC on Wednesday 25 March 2015 concerning Windhoek drinking water quality highlights the need for sustainable development of which the provision of quality drinking water for the current residents of the City of Windhoek and the future residence of the same comes into question. A key question in understanding the concept of sustainable City development is whether City of Windhoek, its Industries and its residents should continue on their current path.  Malthus (1798) population growth theory says that the development of mankind was limited by the pressure the population growth rate exerted on the availability of food resources. According to Malthus, if the rate of population growth exceeds the rate of food production there will be a crisis leading to a catastrophe. This theory can be applied to any resource and in this case water. In some cases the theory has proved to be true. However there have been technological innovations that have prevented the populations around the world from falling into a Malthusian Crisis.

The water treatment system in Windhoek is one of the most novel systems in the world in that it recycles all the sewage water back into the drinking water stream. Because it is located between two deserts, and has no major rivers nearby, Windhoek does not have a reliable source of water. As a result Windhoek was the first city (1968) in the World to implement Indirect Portable Reuse (IPR).   The drinking water treatment system used by the City of Windhoek is a multi barrier system which includes Microfiltration and Reverse Osmosis among other drinking water treatment technologies. Without this innovation the City should have reached the Malthusian Crisis perhaps long back.

In Indirect Portable Reuse (IPR), sewerage water is treated to remove undesirable contaminants such as organic substances, ecoli, nutrients, pharmaceuticals and other chemicals that are present in waste water. This treated water is then blended with ground water and surface water to reduce the risk associated with treated waste water and this may involve discharging the effluent into surface water or ground water. The surface water could be a dam or river which acts as both storage and buffer. In the case of Gorengab dam the buffering effect had been minimised by the low volumes of water prior to the rains. The water can also be discharged into a ground reservoir that is referred to as aquifer recharge. There is always a risk of contaminating the ground water with aquifer recharge however there is also the advantage that comes from further purification as the water filters down the soil profile until it reaches the aquifer. This technique is also used in the Netherlands, where treated wastewater is discharged over the sand dunes to take advantage of the purification properties of sand. The filtered water then replenishes the ground water source. The blended water is then further treated in a drinking water treatment plant to ensure that the water is of an acceptable drinking water quality.

The news article appeared to show green algae forming on the surface of the water in the City’s reservoir. The state of the water as seen on the television may be a sign of an approaching Malthusian Crisis. The capacity of the Water treatment system appears to be strained in some way hence the excessive growth of green algae in the water reservoir. The appearance of green algae could be purposeful as green algae is autotrophic in nature as it consumes carbon dioxide and releases oxygen into the water but it also a form of contamination feeding into the drinking water purification plant.

In water treatment engineering or sanitation engineering water that is discharged from sewerage treatment plants should be of a certain standard in terms of the Biological Oxygen Demand (BOD), Chemical Oxygen Demand (COD), and nutrient concentrations. In layman’s language BOD is a measure of the amount of food that is in the waste water which is consumed by micro-organisms for metabolism and cell growth. Metabolism is an oxidation process which requires the supply of dissolved oxygen. If effluent is discharge into a natural water body with a lot of BOD it cause depletion of oxygen which in turn affect other aquatic forms of life which rely on dissolved oxygen such as fish. The role of a sewage treatment plant is to enhance the conditions for micro-organisms to consume the entire BOD before the effluent is released into the water bodies. This is the role played by trickling filters and oxidation ponds. As the food in the water depletes so also does the population of micro-organism. The nutrients generally constitute of nitrates and phosphates. The concentration of nutrients in the effluent from the sewer treatment plant (STP) should be below a certain threshold which may vary slightly from place to place depending on the situation. It appears the STP is failing to deal with the concentration of nutrients in the effluent and hence the growth of algae in the water. If phosphates in particular are not removed from the effluent to the correct levels then algae will proliferate.

The growth of the city population is around 3% per annum, the City of Windhoek and its residence need to innovate again in order to conserve the finite water resource that we have. Such innovation may involve decentralising some of the treatment processes and let them happen on site. One way to think about it is to decentralise some of the operations so that some of the treatment steps are done on site. This may be the incorporation of digesters in the sewer system for domestic households. Digesters have the ability to convert sludge into valuable methane gas and water. They also destroy ecoli which is abundant in sewer water. Digesters also reduce the amount of suspended solids in sewer water making the effluent more fluid. Decentralising some of the sewer treatment steps would increase the capacity of the total treatment system for the city. It also shifts some of the responsibility to the residents who are generating the same waste material which is probably the way to face a better future for the City of Windhoek.

Biogas sanitation systems can be used as part of an onsite household based decentralised or semi decentralised waste water treatment system. They are generally designed to perform primary treatment (settling of suspended solids) and secondary treatment (BOD and nutrient removal). Up to 90% of the organic matter can be converted to biogas depending on the retention time. This scenario favour the decentralised systems as it is easier to design digesters with a much greater retention time to allow for effective conversion of organic matter to biogas and removal of pathogens. Furthermore part of the cost of increasing the capacity of the sewerage treatment plant can be shifted to the point of generation.

Depending with the conditions and with augmentation of other organic kitchen wastes a person can produce per day between 16 liters and 29 liters of methane which is equivalent about 0.22 kWh of electricity on average. From a generation perspective this translates to about 0.36 kWh per person taking into account generation and transmission efficiencies. An average family of 5.7 could generate 38 kWh per month which could yield some financial gains apart from environmental gains and cleaner drinking water. Over and above this organic fertilisers and soil conditioners could be produced resulting in reduction in the use of artificial fertilisers and their subsequent environmental impacts. However there are also health risks regarding the use fertilisers from digesters particularly because of helmith eggs which need special conditions to destroy them in the digester.

Most biogas systems were constructed in prisons and schools where aesthetics is not very important. From a biogas system operation perspective the normal toilet systems are not optimal because of the ratio of water to faecal matter which should be 1:1 hence low water flash toilets or no water toilets are optimal for biogas toilet systems. From an Architectural perspective, aesthetics is very important and this may be a hindrance to the implementation of Biogas sanitation systems. There is however toilet technology which satisfies both aesthetics and optimal requirements for biogas digester conditions. Aquatron toilet systems were designed to separate faecal matter and liquids through centrifugal separation. This can be a technology that can be adopted by the City of Windhoek for existing buildings and future buildings as part of a decentralised Sewer treatment System which starts from the point of generation.

Over and above these efforts by the City of Windhoek, there is a need for an integrated forum which would involve all stake holders that can effect decisions towards Sustainable City Development. One such forum is the Green Building Council of Namibia. The Green Building Council of Namibia was conceived by the United Nations Development Programme through the NEEEP project which was administered through the Ministry of Mines and Energy. The Green Building Council of Namibia is still in its formative years and is open to participation by all stake holders that can help develop sustainable Cities. At present the Ministry of Mines and Energy, the Ministry of Works and Transport, Several Green Projects in the City, Architects and Engineers are party to this forum. It would be a positive development for other stakeholders such as the Ministry of Environment and Tourism, the Ministry of Local Government, municipal authorities and property developers to be part of such a forum towards a sustainable future for Namibia.

The operation of the Green Building Council of Namibia is to equip construction professionals with the necessary tools to develop and implement Green Building Strategies. Among such tools is the water rating tool which aims at reducing the amount of water used by buildings. The development of the tools requires the participation of stake holders and professionals, implementation on the other end requires construction professionals, developers and contractors.

The Green Building Council of Namibia will be holding its second convention from the 21st to the 24th of April 2015. The convention includes two days of training leading to Accreditation. It is an opportunity for all the relevant people to acquaint themselves with issues pertaining to sustainable buildings.

In conclusion availability of drinking water of a good quality for the present and the future generations is one of the key pillars of sustainable development in Namibia.