Chapter 3 Decision Support Model (IUWS-DSM) - Tubdok

Chapter 2 · New Conception 35
irrigate the restricted green area in the neighbourhood. In some situations, the treatment is
even unnecessary. On the contrary, the larger pipe system can be required. Importantly for
both cases, if the reclaimed water may expose to human beings, it must be hygienic safe.
Indirect potable reuse. It is defined as after being buffered and recovered in nature for
certain period, the reclaimed water is used for raw water supply. Unconsciously, water is
already being reused in such manner. For example, the treated or even untreated sewage
discharged upriver will be used as source water for water consumers downstream. By
treating the used water into very high quality (Quality A(1)), the recovering period of used
water is accelerated, whereby the local area gains more source water, and at the same time
the risks can be better monitored and hereby reduced. Several methods are practised for
indirect potable water reuse, typically groundwater recharge and river bank filtration. The
additional treatment is added to the mixture of raw and reclaimed water before distribution as
drinking water (Metcalf & Eddy 2007).
The reclaimed water can be also directly reused as drinking water without intervening
storage. There are only a few cases of direct potable reuse implemented in situations of
extreme water scarcity, e.g. in Windhoek, Namibia (Du Pisani 2006), However, it is still not
universally accepted by our society and in fact the public often rejects water recycling
activities (Dolnicar and Schäfer 2006). Thus, direct potable reuse is not involved in IUWS.
Source separated collection system (SSCS). For enhancing the efficiency of water
reuse system, one method is to control the pollution sources. Particularly, the urban toilet
system needs to be rethought, as it contains most hygienic dangerous substances. Hence,
SSCS becomes an option for sewer systems.
Since different domestic sewage streams have different water characteristics (see §
2.1.4.1), they should be collected separately, so that they can be better handled with
specified methods. SSCS has significant advantages. First of all, the reclamation and reuse
of greywater become easier, as it contains less hygienically dangerous substances. If
vacuum toilets are installed, plenty of water can be saved. Meantime, valuable nutrients can
be regained such as N and P, and the renewable energy (i.e. biogas) can be generated, as
well.
However, SSCS requires an additional and costly collection system as well as the
additional treatment and storage systems for blackwater (or for brownwater + urine). As a
result, the total system can be quite expensive. Therefore, the SSCS would be the proper
option for water entities having medium or small sizes, whereby the large collection system
can be avoided.
Separate or combined sewerage. Separate sewerage systems transport urban sewage
and storm water in two independent systems, where combined sewerage systems gather
them together in one system. Both types have advantages and disadvantages, and they are
widely implemented. Furthermore, there are more possible types, e.g. the hybrid sewer
system. Which kind of sewer system is better or more appropriate depends very much on the
actual situations, such as city locations, geological conditions, system scales, local economy
level and local regulations, etc. The general priorities and selecting rules are set up
particularly for IUWS and shown in § 3.3.3.