An innovative greywater treatment system for urban areas ... - SuSanA

An important factor for the assessment of a useful greywater plant installation is the
dimension of the building represented by number of persons (N3). The bigger the volume of
treated greywater, the cheaper the price of treated water. That factor was considered as an
important criterion and is weighted with 8 %. The easiest way to estimate this is by the
number of persons living in the building. For the classification, the following ranges have
been fixed: “big number (> 100 persons)” = 10, “medium number (10 - 100 persons)” = 5,
“small number (< 10 persons)” = 1.
The accumulation of greywater (N4) was included in the calculation with a weighting of
10 %. On the one hand it is an appeal to reduce water uptake rate by dual use, in case the
consumption of water as well as the production of greywater is high. On the other hand, the
economic cost calculation is more beneficial when based on high volumes. This makes the
sub criterion very important for the appraisal. A “high accumulation of greywater” (= 10) can
be equalised with an overnight stay e.g. at a hotel and when water from showers and bath
tubs contribute to the overall amount. “Medium accumulation” (= 5) means operation in the
daytime plus showering, such as in factories where the staff takes a shower after finishing
work. “Low accumulation” (= 1) is expected for buildings with operation hours in the daytime,
e.g. in office buildings.
In areas where no sewer system (N5) is available, the removal of sewage is very
complicated. It is determined by high cost- and time efforts due to the collection with tank
lorries. This might be an appeal to reduce sewage water production by means of greywater
recycling. On top, if no sewer system is installed, the operators of large buildings, such as
hotels, have a great interest that no sewage water passes by on the doorstep. Therefore, the
classification is: “no sewer system available” = 10 and “sewer system available” = 1. In total,
the weighting of the sub group is not very high with 2 %, because it is only a contributing
criterion.
The next factor is population or settlement density (N6) in the projected area where the
MBR plant shall be installed. In densely populated regions, problems caused by high
demand of drinking water are more intense than in low populated areas. In addition, the
problems which might be caused by high production of sewage water are more critical, if no
appropriate sewer system is available. It has been assumed that space for treatment plants
is limited in densely populated areas. This aspect makes MBR technology with its little space
requirements highly suitable. Thus, population or settlement density is classified with “high” =
10, “medium” = 5 and “low” = 1. In total, the weighting within the utility analysis is 2 %.
In principle, the installation of a membrane bioreactor makes more sense in an urbanised
area (N7), as it goes hand in hand with the sub criteria of settlement or population density
above. Despite the similarity between sub criterion N6 and N7, both factors were included,
because often there is a high settlement density of hotel complexes far away from urban
areas, and this needs to be taken into account as well. If both criteria apply, there is a high
appeal to use MBR technology. In this respect, the criterion was weighted with 2 % as well.
“High urbanisation” = 10, “medium urbanisation” = 5, and “low urbanisation” = 1, there is no
suggestion of city size added, due to different definitions in every country. In Germany for
example, a city with a population of 100.000 persons is a major city, but a direct comparison
with megacities like Tokyo or Delhi is impossible. Hence, there is the advice to make the
decision based on national definition of evaluated area (Berlin-Institut, 2011).
The main group, namely socio-cultural criteria (S), was estimated with 14% in total for
the appraisal of international transferability. Not only environmental or economic reasons
need to be considered, but also personal and private motives of decision-making.
Regarding general acceptance of greywater reuse (S1), there are different possible
ratings. In Germany the acceptance of recycled greywater for the purpose of toilet flushing is
very high with approximately 93% (Knerr et al., 2009). In Muslim countries, it is often only
accepted to use recycled water for irrigation purposes, due to religious reasons (Sieghart,
2005). The classification was defined with “high acceptance” = 10 for toilet flushing, irrigation,
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