TECHNICALLY SPEAKING
Water reuse offers big wins in securing future water supplies, say Marie Raffin, research scientist at Thames Water Innovation, and Simon Judd, professor of membrane technology at Cranfield University
Water supply of the future I
ncreasing freshwater scarcity has increased both the development and implementation of wastewater reclamation schemes worldwide so as to conserve ever dwindling freshwater resources. Whilst the conventional solution to providing freshwater in arid regions of the world has been to desalinate seawater, it is widely recognised that reuse is more energetically efficient, even when employing membrane technology to provide the same high-quality permeate product. A quick review of the energy demand figures (figure 1) reveals that reuse can incur a significantly lower energy penalty than desalination of seawater, as well as reducing the amounts and concentration of the increasingly problematic waste brine stream. Currently, and commonly in many existing urban areas, unplanned wastewater reuse already takes place: water may be extracted from the water body (such as a river) downstream of the discharge point of a wastewater treatment plant (WwTP) in an unregulated manner (figure 2a). The idea of planned wastewater reuse is to retain part of the WwTP effluent and treat it further via an advanced treatment plant, in which the goal is to achieve a water quality target to allow it to be safely reused.
Reclaimed water from the advanced treatment plant may then be used for number of applications, including industrial process water, indirect potable reuse, direct potable reuse, conservation and/or supplementation of flow in natural environmental water bodies, provision of a barrier against seawater intrusion, non-potable municipal reuse and irrigation.
Indirect potable reuse In the case of indirect potable reuse (IPR) (figure 2b), reclaimed water is injected in the catchment of a drinking water plant, which can be a reservoir, an aquifer or river (for example, Essex & Suffolk advanced treatment plant). Planned IPR already exists and is mainly located in USA, Singapore and Australia (figure 3).
Most of these plants are membrane-based and are either polishing systems, where wastewater already treated by a conventional WwTP is further treated to potable water quality or better, or ‘total’ systems - where the raw
wwtonline.co.uk Table 1. The ten largest MBR WwTPs Table 2. The ten largest membrane-based wastewater reuse plants worldwide, April 2012
sewage is treated directly. While membrane bioreactor (MBR) technology is increasingly used for the latter (Table 1), there remain a significant number of generally much larger polishing plants (Table 2), which are based on a combination of either microfiltration (MF) or ultrafiltration (UF) processes.
In both cases there may be a downstream reverse osmosis (RO) step, and possibly further polishing by UV irradiation, depending on the circumstances and the precise end use of the product permeate water.
A major technical drawback of such membrane systems is the fouling of both the MF/UF and RO membranes, also known as integrated membrane system (IMS). Membrane
fouling reduces the throughput of the process as well as increasing the cost.
The water to be treated by the IMS system varies from one WwTP to another and its
Figure 1 Energy demand u August 2013 Water & Wastewater Treatment 33
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