Sludge characteristics

6.7.2.1. Sludge build-up

Table 36 summarizes the sludge build-up rates observed in the four plants. The largest ABR per capita rate was observed in GB, the lowest in MM. Sasse (1998) cites Garg (unknown year) with a build-up rate of 30 l sludge cap^-1 y^-1 in septic tanks. Further sludge accumulation rates for septic tanks are compiled in Table 18 in Section 4.3.4. All are significantly higher than the rates observed in the ABRs, possibly because of more efficient stabilisation of organic material in the ABR and therefore less sludge build-up. Also, ABR feed is pretreated as opposed to septic tanks of which the raw wastewater certainly contains higher amounts of nonbiodegradable particulates.

Foxon (2009) normalized the sludge accumulation rates observed during her ABR study using OLR. The amount of accumulated sludge per kg COD applied was approximately 2.1 l (kg COD applied)^-1 during a loading regime described as supportive to good treatment and anaerobic digestion. Sludge build-up observed in this study was significantly lower (see Table 36).

Table 36: ABR and per capita sludge build-up rates at the four sites

BWC GB MM ST

Yearly sludge build-up m³ y^-1 0.8 1.7 0.8 2.9

Per capita sludge build-up l cap^-1 y^-1 4.2 8.5 3.1 6.5

Normalized sludge build-up l sludge (kg COD applied)^-1 0.38 0.74 0.19 0.64

6.7.2.2. Sludge build-up distribution

Sludge build-up in GB, MM and ST occurred predominantly in the last reactor chambers (see Figure 134). This trend was observed the more the longer the plant had been operating: over time the highest observed sludge level in all 2 ABRs shifted towards the rear compartments.

In BWC during Phase I on the other hand most build-up occurred in the first compartment and gradually decreased towards the rear of the reactor (see Figure 134). The closer a chamber was to the feed, the higher its sludge level was during all 2 y of Phase I.

Foxon (2009) reports that right after start-up of a pilot ABR in South Africa sludge accumulated most in the first compartments. As the operation progressed, accumulation was also observed in later compartments so that, similar to the Indonesian case studies, sludge levels there eventually exceeded the levels of the first chambers.

Figure 134: Fractions of ABR sludge build-up observed in the different reactor chambers

Figure 135: Average ABR settled sludge TS and VS concentrations at the four sites, error-bars indicate standard deviations of measurements across chambers

6.7.2.3. Sludge Total and Volatile Solids concentrations

Settled ABR sludge TS and VS determinations are expected to have considerable uncertainties associated to them since they involve a number of measurements that are prone to error: sludge level measurements, sludge sample volume measurements and TS and VS concentration measurements.

Nevertheless the results provide a coherent picture across all four plants. They all indicate higher sludge (and especially TS) concentrations in the first and approximately constant concentrations in all following reactor chambers (see Figure 136 and Figure 137):

 In all four plants higher TS concentrations were found in the settlers than in the ABR chambers.

 In two plants (BWC, MM, ST) the highest ABR-TS concentrations were measured in the first ABR chambers.

 In the case of BWC and MM, the highest VS concentrations were observed in the settlers and first ABR chamber. In the other plants the VS concentrations were approximately constant throughout the ABRs.

Average TS concentrations of ABR sludge varied across the systems from about 50 g TS l^-1 to 95 g TS l

-1. The sludges from all four ABRs had a similar average VS concentration of about 30 g VS l^-1 (see Figure 135). Mtembu (2005) observed settled sludge TS concentrations of 12 g TS l^-1 to 34 g TS l^-1 on a pilot ABR in South Africa. Foxon (2009) reported an estimated VS to TS ratio of 0.57 on that same plant which results in a settled sludge concentration of 7 g VS l^-1 to 19 g VS l^-1. This is significantly lower than values observed in the four case studies, which apparently featured a much more dense sludge.

Koottatep (2014) reported TS concentrations of thickened bottom sludge in onsite sanitation systems treating raw sewage of 40 to 220 g TS l^-1. VS content of this sludge was 60% to 70%. The significantly lower VS content of the sludge observed in the four case studies (52%, 42%, 55% and 39% in BWC, GB, MM and ST respectively) may be due to better stabilisation.

0%

Fraction of sludge build-up in whole ABR

ABR 1 ABR 2 ABR 3 ABR 4 ABR 5 (& 6)

Figure 136: Settled sludge TS concentrations at the four case study sites

Figure 137: Settled sludge VS concentrations at the four case study sites

6.7.2.4. Methanogenic activity

Figure 138 and Figure 139 present the SMAmax values measured during the wet and the dry season at the four case study plants and compare them to the benchmark value proposed by Pietruschka (2013) for active anaerobic sludge.

Most methanogenic activity was found in MM where it even reached the benchmark value proposed by Pietruschka (2013). Least methanogenic activity was measured in GB. Also there does not appear to be a correlation between the amount of bubbles found on the chamber supernatant and the corresponding SMAmax value although more data would be needed to confirm this. SMA measurements done in BWC cannot be compared to the other plants since the sludge height measurements indicated sludge washout from the digester into the ABR. Such washout would certainly have included active sludge. The measured sludge activity in the ABR can therefore not be solely attributed to ABR operation.

The existing SMA dataset is not based on a very large number of measurements which was at first regarded as limiting its general informative value. However a number of recurring observations can be made across the plants. It is argued that this coherence justifies a certain confidence in the data although some of the following interpretations will certainly have to be verified by further investigations.

First of all, system components with low methanogenic activity are strikingly similar across case studies:

In both plants built without digester (MM and ST) settler sludge yielded very low SMAmax values indicating low fractions of active methanogenic MOs. Similarly low SMAmax values were observed with all sludges sampled from rear ABR chambers and all AF chambers.

0 50 100 150 200 250

BWC GB MM ST

g TS l-1

Settler 1 Settler 2 ABR 1 ABR 2 ABR 3 ABR 4 ABR 5 ABR 6 AF 1 AF 2 AF 3

0 25 50 75 100

BWC GB MM ST

g VS l-1

Settler 1 Settler 2 ABR 1 ABR 2 ABR 3 ABR 4 ABR 5 ABR 6 AF 1 AF 2 AF 3

On the other hand there is a general tendency of the highest SMAmax always being in one of the first three ABR chambers, especially during the dry season.

Sludge activity is generally found to increase after a period of approximately 40 d without rain influence. Although this is not the case for all reactor chambers, the occurrences of measured sludge activity increase outweigh the cases in which the sludge activity did not increase.

It is acknowledged that the unknown MO fraction of two sludges with similar VS concentrations makes it impossible to differentiate between non existing methanogens and existing but inactive methanogens (see Section 3.4.6). An observed difference in SMAmax values therefore only allows making a qualitative comparison on the average acetoclastic methanogenic activity, not on the amount of methanogens per se.

It is hypothesised that the two main causes for the above mentioned variations of acetoclastic methanogenic activity in ABR chambers are: substrate availability and forced migration through flow surges.

It is striking that SMAmax values always indicate alternating activity strength across chambers in MM, ST and BWC: chambers with high activity sludge are always followed by one or two chambers with significantly lower activity in all measurement campaigns.

Such a pattern could be explained by the varying availability of easily biodegradable substrate: high substrate availability in one chamber would lead to an activity increase of the MO-consortia feeding on this substrate (in this case the acetoclastic methanogens). The resulting high substrate uptake would lead to the reduction of available substrate for the MO population in the following chambers, therefore reducing their activity. This reduced activity of consuming organisms allows the build-up of easily biodegradable substrate which is then available for the MO in the following compartment and the process starts anew. This would imply in all four plants hydrolysis being the rate-limiting step since substrate availability for methanogens appears to be too low to sustain a high biomethane activity throughout all chambers.

Low substrate availability also appears to be a plausible explanation for the low activities of settler and AF sludge. It is possible that incomplete hydrolysis processes inside the settler may not have enabled a large methanogenic community to develop. Most released substrate is then consumed throughout the ABR, starving the populations in the sludge at the bottom of the AF chambers.

Sludge level measurements in MM, GB and ST showed the occurrence of sludge migration from the first to the last reactor chambers. This migration however does not appear to have affected the average methanogenic activities proportionally. If that had been the case most biomethane activity would have been found in the rear of the ABR and in the AF.

The fact that the front ABR chambers contained the sludges with the highest SMAmax values, even at the end of the wet season, leads to the conclusion that active acetoclastic methanogens succeeded in establishing a stable community even under high hydraulic loading. Acetoclastic methanogens appear therefore to be surprisingly resilient to washout. Their marked activity increase after 40 d to 60 d without rain (especially in the front chambers) indicated that they certainly were impeded during the wet season. Whether the storm water primarily affected the sludge because of reduced substrate availability or through washout of methanogens cannot be determined with the available information.

The data however tends to point towards the reduced substrate availability as being the cause.

Figure 138: SMAmax values measured across reactor chambers of the four case study plants during wet-season

Figure 139: SMAmax values measured across reactor chambers of three case study plants during dry-season