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

Chapter 3 · Decision Support Model (IUWS-DSM) 69
If used water is collected in UD, it is not suggested transferring used water further to CA
in order to avoid large sewer and lift stations. The treatment plant is necessary if used water
is disposed in UD. Due to its severe conditions, the used water treatment plant can have
strong impact on its surroundings. On the other hand, it can be the advantage because it
forces the city planners to plan the urban area in a more volumetric and natural way.
Regarding four system options in UD, there are three kinds of priority sequences based
on the types of collection system. If the SSCS is applied, local discharge is considered in the
first, and then two types of water reuse in UD, and at last diversion of used water to CA. This
is because greywater can have better hygienic conditions than urban sewage. In the second
kind of sequence, the SSCS is not used, whereby the bad quality of urban sewage results in
that both water reuse options have lower priorities than diversion of used water to CA. The
third kind of sequence is for the combined sewerage system without SSCS. As the
conventional system, transporting used water to CA is considered first, then local discharge
in UD, followed by two water reuse options. In principle, indirect potable reuse has higher
priority than direct non-potable reuse because of the dual water system.
WUU has three system options of used water. There are two kinds of priority sequence
depending on whether the SSCS is applied and regardless of the types of the collection
system. If greywater is gathered, self direct non-potable reuse is mostly considered in the
first, and then direct reuse to neighbours. Diverting greywater to UD is the last option. If
urban sewage is collected, the priority order is just reversed. Either combined or separate
sewerage system plays no important role in determining the system options of used water in
WUU.
Blackwater is the additional and independent system that is attached to the greywater
system. If the SSCS is applied, the blackwater system is automatically involved. Usually, two
system options are suggested, i.e. either store or anaerobic digestion. The item 11 in Table
27 is the recommended condition to build up blackwater treatment plants.
All priorities are set up for general conditions. Eventually, the system options are determined
based on the actual situation of projects.
3.3.5 Rainwater
In principle, rainwater should be retained in its initial place and supplemented to groundwater
afterwards. Following the characteristics of rainwater, the utilisation systems should be
also scattered. Rainwater should be assimilated into the nature mostly in WUU. The
groundwater recharge of treated rainwater or the groundwater injection is suggested in UD
due to economic scales of treatment and injection facilities. Disposing rainwater in CA should
be avoided by all means in order to avert the huge storm drain and lift stations.
In the same way, by integrating Table 26, Table 27, Figure 21 and Table 12, the priorities
of system options for rainwater management are set up in Table 29. Since CA has only
one option to deal with rainwater, i.e. discharge to local surface water body, it is not listed in
Table 29. Though it is not recommended option, it can be the suitable solution in some
particular situations, e.g. city is located on the hillside.