44 ENVIRONMENT AND ENERGY
Many large cities in developed and transitioning economies have extensive sewerage systems, some of which are still functioning effectively some 100 years after construction. or pervious surfaces capture rainwater before it runs onto polluted pavements and streets. These solutions can alleviate peak flows, minimise the risks of urban floods and pollution, and reduce the need for investments in additional hard infrastructure and treatment facilities. They can attract private investments, encouraging property and land developers to invest in new buildings equipped with localised drainage systems.
This may require changes in local by-laws, as local regulations will, to a great extent, dictate the final choice. On the other hand, decentralised stormwater drainage only offers a solution for temporary retention, as the water will ultimately need to be transported to sewer systems. In some cases, the maintenance costs will be higher, but decentralised systems help to attain benefits like improved human wellbeing, absorption of air pollution and moisture retention, thus lowering ambient temperature and attenuating the urban heat island affect, ultimately contributing to the greening of cities.
Decentralised systems can also be used for the treatment of runoff from highways. Experience accumulates with the implementation and exploitation of decentralised sanitation and urban drainage. Nonetheless, some barriers have to be overcome, such as social perceptions and difficulties associated with retrofitting. situation or by other factors like global warming, water scarcity, environmental quality issues and / or land use planning. In the rapidly urbanising centres worldwide, the prevention of the discharging of carbonaceous material was the priority in order to protect receiving waters being starved of oxygen.
The oxygen demand was‘ satisfied’ by using large amounts of energy to encourage the growth of microbial biomass( sludge), which was separated from the system and used in agriculture or dumped at sea. Later developments saw extended aeration systems to reduce the final amount of biomass for disposal, as this was responsible for a large proportion of the treatment costs. During the oil crisis in the
An additional challenge is the need to manage wastewater at different scales( from buildings to the municipal level, to even larger levels). These barriers can be overcome by a combination of information campaigns, a whole-of-government approach to urban water management( including policies, laws and regulation), business models for water utilities and land development that factor in externalities related to wastewater management, and a long-term vision of the challenges in the water sector and the opportunities for urban development.
EVOLUTION OF TREATMENT TECHNOLOGIES Significant advances have been made in treatment technologies, since the original development of aerated systems( e. g. activated sludge and trickling filters) during the 1920s. The selection of treatment systems has been driven by the prevailing economic
August 2018 Volume 24 I Number 6 www. plumbingafrica. co. za