Wastewater: flows, management, and as resource

By Extracted from the United Nations World Water Development Report 2017

Wastewater flows are as varied as its sources and the types of constituents they contain, with the latter being a function of the former.

Wastewater in water cycleFigure 1: Wastewater in the water cycle.
Image credit: WWAP

Figure 2 provides an overview of the main wastewater flows, from their generation at the source to their ultimate fate. Uncollected wastewater (and all its constituents) ultimately ends up in the aquatic environment. This is also the case for wastewater that is collected and disposed of without treatment, the proportion of which can in some cases be considerable.

Wastewater flowsFigure 2: Wastewater flows.
Image credit: WWAP

table 1Table 1: Examples of negative impacts of untreated wastewater on human health, the environment, and productive activities

Wastewater treatment can allow for the separation of water and other constituents, which can then be reused or disposed of.


Controlling and regulating the various wastewater flows is the ultimate purpose of wastewater management. The wastewater management cycle can be broken down into four basic interconnected phases or steps:

  1. The prevention or reduction of pollution at the source, in terms of pollution load and volume of wastewater produced.
    Prohibiting or controlling the use of certain contaminants to eliminate or limit their entering into wastewater streams through regulatory, technical and/or other means. This step also includes measures to reduce the volumes of generated wastewater (e.g. demand management and increased water use efficiency).
  1. The removal of contaminants from wastewater streams.
    Operational systems (including collection infrastructure) and treatment processes that remove various constituents of wastewater (i.e. contaminants) so that it can be safely used or returned to the water cycle with minimal environmental impacts. There are several types and levels of wastewater treatment, the choice of which is dependent on the nature of the contaminants, the pollution load and the anticipated end use of the effluent.
  1. The use of wastewater (i.e. water reuse)
    Safe use of treated or untreated wastewater under controlled conditions for beneficial purposes. Historically used primarily for irrigation, wastewater treatment technologies have now advanced to allow for the use of treated wastewater for other uses, provided that the level of treatment and the quality of the effluent are ‘fit for purpose’.
  1. The recovery of useful by-products
    Various constituents of wastewater can be extracted, either directly (e.g. heat, nutrients, organic matter and metals) or via supplementary transformation processes (e.g. biogas from sludge or biofuels from microalgae). There is a growing number of potentially cost-effective opportunities for extracting useful materials from wastewater, such as nitrogen and phosphorus, that can be transformed into fertiliser.

An additional role of the wastewater management cycle is to mitigate any negative impacts on human health, the economy and the environment. When taking into account the multiple benefits of improved wastewater management, several of these processes can be considered cost-effective, thus adding value across the wastewater management cycle while supporting the further development of water supply and sanitation systems.

Based on the assumption that it is possible to align water quality requirements with water use locations, multiple use systems with cascading reuse of water from higher to lower water quality can make water reuse more affordable than providing extensive water treatment at each point of abstraction along a river basin (UNEP, 2015c).


In practice, the goal is to go beyond mere pollution abatement and to seek to gain value from wastewater, if for no other reason, as an additional means of paying for wastewater management and for enhancing the economic sustainability of the system.

However, wastewater management is already an important part of several different resource cycles and is well positioned to play a central role in the circular economy. Using appropriately treated water for agriculture and power generation enhances opportunities for food and energy security, and can help alleviate the stresses brought about by increasing demand for water. This will have positive repercussions on freshwater supplies, human and environmental health, income generation (livelihoods) and poverty alleviation.

Furthermore, water reuse can generate new business opportunities and support the advancement of a ‘green’ economy. Aquatic ecosystems (e.g. ponds, wetlands and lakes) offer additional, low-cost solutions for enhancing wastewater management, provided they are managed sustainably. Although planned use and functional markets of wastewater for ecosystem services is a relatively recent phenomenon, the valuation of treated wastewater use for ecosystem services reveals favourable environmental and economic benefits.

The informal use of untreated wastewater is already occurring widely, out of simple convenience or as a matter of sheer need, and all too often in the absence of appropriate safety control measures. While measures that promote the direct use of certain types of untreated wastewater may be relatively easy to implement, the cost of developing treatment systems for recovering wastewater from certain specific human activities may be prohibitive in some cases.

There can also be a mismatch between the location and timing of the source of wastewater, and its eventual use. Wastewater management systems, therefore, need to be designed based on its characteristics (e.g. origin, components and level of contaminants) and the intended end use of the effluent stream, including any useful by-products, as these will dictate the most appropriate and practical wastewater source.

Table2Table 2: Framing wastewater management from a resource perspective

There are strong economic arguments in favour of optimising freshwater-use efficiency, managing wastewater as a resource and eliminating (or at least reducing) pollution at the point of use. Utilising wastewater at, or as close as possible to, its source generally increases cost-efficiency due to the lower costs of conveyance. The fact that so little wastewater management is currently occurring, particularly in developing countries, means that there are vast opportunities for water reuse and for the recovery of useful by products, provided the appropriate incentives and business models are in place to help cover the substantial costs.

Recent market studies also show that there is a positive trend in water and wastewater treatment investments in developing countries. Worldwide, the annual capital expenditures on water infrastructure and wastewater infrastructure by utilities have been estimated at USD100-billion and USD104-billion, respectively (Heymann et al., 2010). Since wastewater management is implemented at the local level, responses and technical solutions will need to be location-specific. In this respect, there are opportunities in further integrating wastewater management, including sanitation and faecal sludge management (FSM), with water resources and solid waste management. This requires governance structures that foster collaboration across institutional boundaries, as well as accountability and compliance with regulations for wastewater use and the extraction/use of recovered by-products.

Wastewater treatment can allow for the separation of water and other constituents, which can then be reused or disposed of.

Above all, wastewater management needs to be planned from ‘upstream’, at the source, in order to complement end-of-pipe solutions ‘downstream’. A number of pressures on water resources are driving the need for the enhanced use of wastewater. Population growth, urbanisation, changing consumption patterns, climate change, loss of biodiversity, economic growth and industrialisation all have an impact on water resources and wastewater streams, with repercussions on atmospheric, land and water pollution.

An improved approach to wastewater management will help alleviate the impact of some of these pressures. From a resource perspective (see Table 2), sustainable wastewater management requires: i) supportive policies that reduce the pollution load upfront; ii) tailored technologies that enable fit-for-purpose treatment to optimise resource utilisation; and iii) taking account of the benefits of resource recovery.

Such a perspective promotes the implementation of innovative financial mechanisms, while embracing a precautionary approach and the polluter-pays principle. It is the responsibility of national governments to provide the policy environment for equitable tariff structures that help ensure the operation and maintenance of existing infrastructure, and attract new investments along the wastewater management cycle.

To be continued in future editions of Plumbing Africa.

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