Environmental Biotechnology - Theory and Application

140 Environmental Biotechnology

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One particularly interesting example, put forward by the University of Rhode

Island, involves utilising the intrinsic abilities of bivalve molluscs such as clams,

mussels and oysters, to act as natural biofilters of estuarine waters. These species

derive most of their nutritional requirements by sifting the water, actively discrimi-

nating between particles, ingesting food while depositing rejects onto the substrate.

There is also evidence to suggest that microbial and chemical processes in the

sediment convert a number of substances present in this matter to less biologically

active forms.

Increasing the local population of suitable shellfish, possibly in artificial aqua-

culture units, could contribute to a significant reduction in turbidity and improved

removal of nitrogen and other nutrients. One of the main advantages of this approach

would be that the waters are continually cleaned and at little or no cost. In addition,

the system might also be harnessed to offer a boost to the local economy, providing

a crop to be harvested or potentially supporting larger commercial ventures.

Even for areas where it would be inappropriate to establish intensive shell fisheries

of this kind, initiatives to restore and conserve indigenous mollusc species could

prove a cost-effective complement to conservation projects or coastal management

strategies, especially those designed to mitigate the impacts of eutrophication.

Case Study 6.2 Flexible Sewage Treatment (Takoradi, Ghana)

Often one of the key features of biological effluent treatment is its simplicity. As the

Takoradi raw sewage treatment plants in Ghana show, for many parts of the world,

when resources or skilled labour are in short supply, this can be a major advantage.

The work, which involved dealing with the effluent from military barracks,

was carried out by Naston of Weybridge, England. Both of the systems supplied

were designed on the basis of a population equivalent of 5000, with a daily

throughput of up to 1million litres. Each site was equipped with twin aeration

tanks 19metres in diameter, containing integral settlement chambers, aeration

and excess sludge removal systems, together with associated 6metre diameter

sludge-holding tanks. The use of two treatment cells, rather than a single, large

one, enables the plant to be operated for maximum efficiency, allowing one of

them to be shut down if the population falls below half its design capacity for

extended periods.

Since both facilities are in remote areas and were built under supervision by

unskilled labour, the design deliberately minimised the civil engineering require-

ment. Other aspects of the plants were, likewise, kept simple to ensure that the

system is easy to operate, involving no daily maintenance and little more than a

routine periodic servicing to the pumps and aerators.

Thus the wastewater collects at the pumping facilities and is transferred directly

to the aeration tanks, where it is processed by the rich complement of micro-

organisms growing in the well-oxygenated environment. After the aeration phase,

the mixed liquor of treated sewage and active microbes is transferred at a constant

rate to the clarifiers, which helps to achieve a good separation and the resulting

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