Plant-feed characteristics

6.7.1.1. Users

Figure 131 compiles the aforementioned plant user connection numbers and puts them into relation with design assumptions. Variations over the years and inaccuracies in the estimations can of course not be excluded.

Operators and heads of communities were however generally very well informed about the dynamics and events in their communities and always showed interest in sharing information needed by the research team. Also all four communities were in residential areas

with constant numbers of residing families where no large variation in population numbers is to be expected. Pipe systems were checked for major blockages and breakages by the research teams in India and Indonesia. Where broken house connections were found, the number of connected persons was adjusted accordingly.

It is therefore assumed that potential variations not reflected by the data did not exceed a level that would significantly affect conclusions drawn further below.

The user numbers in BWC and GB were very close to design assumptions. The actual population size connected to the plants in MM and ST was about 70% and 130% of the values anticipated respectively at the design stage.

6.7.1.2. Flow

Figure 132 presents the average diurnal flows measured at all four sites. The averages presented for BWC were calculated with all data measured after the flow reduction in 2011 mentioned in Section 6.3.4.2. The MM curve was computed with the data from both measurement campaigns of which, for reasons unknown, the second yielded higher flows (see Section 6.5.4.2). This explains the comparably high standard deviation of measured daily flows presented in Figure 133. The GB and ST curves are based on data from one measurement campaign each.

Diurnal flows recorded in GB, MM and ST all featured morning and evening peaks typical for communal wastewater (Haestad et al., 2004). The community in BWC only received water in the morning which explains the non-typical diurnal fluctuation pattern measured there. The peak factors were 2.0, 1.8, 2.3 and 1.8 for BWC, GB, MM and ST respectively and therefore tend to confirm the assumed design peak factor of 2 (also see Section 4.3.1.2).

Average daily plant flows were similar to design assumptions in the case of GB, MM and ST (see Figure 133). The actual flow to BWC however was about 50% of what was projected during plant design. The resulting average per capita wastewater production was extremely low with only 30 l cap^-1 d^-1. The highest average per capita value was measured in MM with 109 l cap^-1 d^-1.

Figure 131: Number of connected users per plant

0

Figure 132: Average diurnal flows measured at the four case study sites

Figure 133: Daily flows at the four sites, error-bars indicate standard deviation across measurement days

6.7.1.3. Raw-water alkalinity

Well water alkalinity was measured six times, once, twice and once in BWC, GB, MM and ST respectively. The measurement results from the four sites were 468 (SD= 59), 132, 126 and 180 mg CaCO3 l^-1 respectively.

These values, although representing very few measurements, allow a valuable approximation of the average raw-water alkalinity in the four communities given that fresh water alkalinity from the same source depends primarily on geological factors and therefore varies little. All values measured in Yogyakarta were similar which appears plausible since most Indonesian households use low depth wells (WHO/UNICEF, 2013) therefore accessing similar aquifers.

Foxon (2009) hypothesised that the low ABR treatment observed during her pilot scale investigations was, amongst other reasons, caused by too low wastewater alkalinity (approximately 200 mg CaCO3 l

-1). This led to a low reactor pH often below pH 6.5 therefore reducing biochemical conversion rates and microbial growth which in turn reduced the maximum up-flow velocity at which the system could be run. From a steady state modelling exercise, the author concluded that a feed alkalinity of 1,000 mg CaCO3 l^-1 would be needed to guarantee a process pH of at least 6.5.

Field data from the Indonesian case studies presented here however suggests that this does not apply to their situations. Raw-water alkalinity from bore wells was about 150 mg CaCO3 l^-1 at all sites and increased to 300 (MM) and 400 (GB and ST) mg CaCO3 l^-1 in the reactor feeds. The wastewater alkalinities throughout the systems remained approximately constant at these values. As opposed to Foxon’s observations at slightly lower alkalinity, median pH values were approximately 7.0 in all plants and minimum values very rarely dropped below 6.5.

6.7.1.4. Estimated plant feed concentration, per capita COD production and pre-treatment efficiency The average pre-treatment HRTs were approximately 73 h, 27 h, 10 h and 13 h for BWC, GB, MM and ST respectively. Even when taking into account that accumulated sludge reduces the active reactor volume and therefore also the HRT, these values are significantly larger than the 2 h HRT suggested by Sasse (1998).

Plant feed concentrations are difficult to measure and the available data was not considered to be sufficiently representative to extract information from. The plant feed concentrations can therefore only be estimated by either:

1. assuming the per capita COD production of the connected populations (of which the sizes and wastewater productions are known). The typical literature value is 120 g COD cap^-1 d^-1

Table 35 summarizes the results of both these approaches. The results indicate in all cases that, when assuming correct population numbers, both assumptions cannot hold at the same time: the pre-treatment efficiency may have been higher and the per capita organic load may have been lower than the typical literature values.

Biogas production rates measured at the BGD in BWC support this since they indicate a probable COD removal of more than 76% and a per capita COD production of slightly above 60 g COD cap^-1 d^-1 (see Section 6.3.8.3). However, COD removal rates of simple settlers above 60% appear unrealistic when comparing with literature (see Section 2.1.2.2), especially when considering that the settlers in MM and ST had only about 12 h HRT. In these cases, lower per capita organic loads (such as suggested by Campos and vonSperling (1996)) become more plausible (see Section 2.5.4).

Extensive feed concentration measurement campaigns would be needed to further investigate this question.

Table 35: Summary of plant feed concentration assessments

Calculation method 1 Calculation method 2

Plant Per capita

Increased flow during rainfall was measured in BWC and MM (see Section 6.3.4.2 and Section 6.5.4.2).

Signs of strong water level fluctuations inside the ABR chambers were observed in GB, MM and ST.

Such high water levels had never been observed during times of peak flow on dry weather days.

In MM the head of community even reported that during extremely strong rain the system regularly completely filled with water to a point where water was pressed out of the closed manhole covers.

This phenomenon did not occur in any of the other systems.

It is concluded that such water level fluctuations had to be caused by flows significantly greater than normal peak flows. It is argued that only water infiltration during storm water events can have led to

this. Thus, all four plants were exposed to unknown but certainly considerable peak flows during wet seasons.