Decentralised approach to wastewater treatment is the handling of wastewater close to the
point of generation. This concept which employs onsite or clusters system is gaining more
relevance in the developing world as opposed to the centralised wastewater treatment regime
applied in industrialised nations. However, decentralised wastewater treatment is not without
its share of challenges resulting from choice of inappropriate technology, improper siting of
infrastructure, inadherence to correct design concepts and lack of proper maintenance. These
bring about negative public health and environmental impacts including groundwater nitrate
contamination, eutrophication of surface water bodies and contribution to global warming
through the emission of green house gases.
This work reviews some existing technologies applied in decentralised wastewater treatment
and disposal in developing countries such as the septic tank, imhoff tank, soil infiltration
device etc. Technologies that are potentially sound and beneficial for wastewater treatment
are also explored. Emphasis is placed on fecal sludge management which is grossly
inadequate and in some cases totally lacking in rural, peri-urban and some urban areas of
developing countries.
A case study is undertaken for a portion of Ezenei Quarters, Asaba, an urban settlement in
Nigeria, West Africa with a population of about 1656 persons. This study area is
representative of many suburban sprawls in developing countries in terms of wastewater
treatment, population size and economics. An attempt is made to select the most appropriate
combination of decentralised technologies to improve on the present wastewater handling.
These technologies will demonstrate the importance and feasibility of recovery and reuse of
organic waste in an area with no wide spread application. The system proposed uses the
combined advantage of both black and greywater for effective treatment. The combined
wastewater is however treated in sedimentation tanks in series (70 m
3
and 60 m
3
respectively)
for the settling and eventual discharge of wastewater sludge. The effluent from the
sedimentation tanks is directed to a combined intermittent sand filter and vegetated subsurface
wetland system which are both 500 m2
in area The intermittent sand filter is aerobic in
function aiding nitrification while the subsurface wetland in anaerobic with enhanced
denitrification. Theoretical mass balance shows discharge quality to groundwater meeting the
design limits of less than 5 mg/l BOD, TSS, and N, and 1 mg/l P.
The sludge withdrawn from the sedimentation tanks is digested in a 50 m
3
anaerobic digester.
The anaerobic digester is also fed with the organic fraction of municipal solid waste including
vegetable waste from the nearby Ogbogonogo Market. There is an expected yearly methane
production of about 30,000 m
3
, 37% of which can be utilised in a generator in the production
of electric power that can meet the yearly electricity consumption of about 112 000 KWh.
There will be excess biogas meeting about 20% of the cooking energy requirement of the
study area or more if electricity generation is cut down to provide for the area only when there
is power outage from the national grid. The biosolids from the anaerobic digester is further
composted in a 30 m
3
invessel composting unit with sawdust as bulking agent. The resulting
compost is sold to farmers in nearby villages. The total treatment of wastewater incorporates
also rainwater harvesting and treatment in the study area due to the inadequacy of fresh water
supply from the public agency. The harvested rainwater can effectively meet 45% non potable
water use such as toilet flushing, car washing, gardening in the study area.
ii
The benefits of the proposed system include effective and improved sanitation, harnessing of
energy from different biowaste to meet communal electricity and cooking needs, protecting
public health, also groundwater sources and surface water bodies from contamination, the
reduction of green house gases and over dependence on kerosene from fossil origin. Others
include reducing the amount of solid for landfilling and providing freshwater savings.
However, for sustainability of the system, there is the necessity to promote communal
involvement. It is also important to educate the community about the system, its operation and
advantages, informing them in addition on the need for constant maintenance. The responsible
management entity for such decentralised facilities should be the centralised government in
this case; the state government which should conduct regular inspection ensuring that best
management and operational practices are kept. The government can also provide subsidies
for capital cost through a relevant ministerial agency. The major effect to the community in
the implementation of the decentralised wastewater treatment improvement is the need to
construct new pipelines for wastewater collection and the cost involvement which hopefully
can be offset by the energy production and sale of compost. Further studies can however be
carried out on cost implication of the proposed system and the contribution of the present
septic system to groundwater contamination in the area with the view of remediation.