Physical parameters and sludge characteristics

3.4.1. Precipitation

Precipitation in the tropics can occur very locally and should therefore ideally be conducted right on site. Daily precipitation data was gathered with a simple pluviometer consisting of a covered bucket with a funnel of known diameter connected to the lid.

3.4.2. Biogas production and CH

biogas content from biogas digesters

Digester biogas production was measured by connecting biogas meters to the digester gas outlets. The specifications of one of these meters were: “Make: Krom/Schroder Make; Model: MAGMOL BK-G4, 2006; Max. flow: 6 m³ h^-1; Min. flow: 0.04 m³ h^-1; Pmax: 0.5 bar; Temperature range: - 20°C to +50°C”.

The CH4 content of biogas was estimated by measuring the CO2 biogas fraction with a “Brigon Testoryt”

and assuming all other gas fractions to be negligibly small. The accuracy of this estimation was checked through measurements performed by external laboratories.

3.4.3. Interpretation-criteria for assessment of storm-water exposure

Criteria for exposure of an ABR to storm water were observations such as sludge on partition walls or on down flow pipes as shown in Figure 14.

Figure 14: Criteria for exposure to storm water, side view of two ABR chambers

Sludge

3.4.4. Determination of sludge levels and sludge sampling

Sludge heights were measured with a specially devised Plexiglas core sampler (see Figure 15) by first immersing the metal rod with the bottom plate in the reactor chamber. The Plexiglas tube is then lowered onto the metal rod and screwed on tight. The sampler is extracted from the chamber to measure settled sludge heights after 5 min of settling time.

The content of the core-sampler is then decanted to remove most wastewater from the sample. The exact sample volume after decanting is recorded in order to determine the dilution of settled sludge by wastewater. All solid determinations and activity tests are done with homogenised aliquots of these samples.

Sludge accumulation rates were calculated through linear regression of total sludge-volumes in ABR chambers over periods undisturbed by desludging events.

3.4.5. Sludge Total Solids (TS) and Volatile Solids (VS) measurements

Total Solids (TS) and Volatile Solids (VS) sludge measurements are done following APHA (1998). All measurements are performed in triplicate with a standard deviation of triplicates generally below 10%.

Results with higher standard deviations are reported as such. The TS and VS-concentration of settled sludge is calculated using the dilution factor determined when sampling the sludge (see point above).

3.4.6. Specific Methanogenic Activity (SMA) measurement

The Specific Methanogenic Activity (SMA) test investigates the acetoclastic methanogenic activity of an anaerobic sludge by measuring the amount of CH4 produced by a known amount of sludge (expressed as VS) under ideal substrate (acetic acid) saturated conditions. It is expressed as „ml CH4

(as COD-equivalents⁷) g VS^-1 d^-1“.

Acetoclastic methanogenic activity accounts for up to 70% of the methane production in the anaerobic digestion of communal wastewater and for most of the conversion of COD (Seghezzo, 2004). Since methanogenesis represents the last and often most sensitive step in the chain of anaerobic digestion processes, the SMA of a sludge is often used as an indicator for its general anaerobic activity (Souto et al., 2010).

7 The factor fbg which represents the COD value of wet CH4 volume unit at 20°C is 1/385 g COD ml CH4-1 (Soto et al., 1993).

Following the Ideal Gas Law, this leads to a factor of 1/445 at 28°C and 950 m altitude which is representative for measurements in Bangalore and of 1/396 at 28°C and 0 m altitude which is representative for measurements in Yogyakarta.

Figure 15: Schematic depiction of the sludge core sampler as used in this study, cross section of a reactor chamber containing sludge

The informative value of the SMA test is however reduced by the normalization to VS because VS does not differentiate between dead organic material and methanogenic MO biomass. Different sludges with similar VS concentrations could therefore contain different amounts of methanogenic MOs. As a result it is impossible to differentiate between non existing methanogens and existing but inactive methanogens only based on the SMA value. An observed difference in SMA values therefore only allows a qualitative comparison on the average acetoclastic methanogenic activity, not on the amount of methanogens per se.

The substrate used in all SMA tests was sodium acetate since it has pH-stabilizing properties as opposed to acetic acid of which the addition to a solution would lead to significant pH reduction.

Following Soto et al. (1993) maximum SMA (SMAmax) should be determined on the linear section of the cumulative methane-production curve during the first hours of the experiment, when VFAs are still high (see Figure 16). The reaction kinetics are therefore substrate saturated and the influence of other processes can be considered negligible.

Consequently it is crucial to ensure the correct substrate to inoculum ratio during the tests in order to produce representative data. Too little substrate would lead to non-saturated conditions or a too short phase of non-saturated conditions. Too much substrate on the other hand would shock the sludge (Pietruschka, 2013) and lead to a lag-phase during which the MO’s adapt and little or no methane production occurs (see Figure 16).

Cho et al. (2005) defined the SMAmax as the peak on a SMA vs. time plot (see Figure 17). This is the value used in this study to compare different SMA results across reactor chambers and plants. Only the first 5 h of methane production were considered to determine the SMAmax value of a sludge (see Figure 17) since potential later peaks could be due to acclimatisation of the sludge to the substrate.

These peaks would not represent the state of the sludge when it was sampled.

Figure 16: Idealised representation of typical CH4

production curves under substrate saturated, non-saturated and over-non-saturated conditions, the dotted mark shows the curve section indicating substrate saturation.

Figure 17: Showcase data to illustrate the SMAmax

value determination, coloured area indicates the five first relevant hours of the test

There is no existing standard SMA method and methods mentioned in literature vary considerably (Souto et al., 2010). Pietruschka (2013) proposed a methodology for DEWATS-sludge adapted to research conditions in developing countries that was further refined and tested as part of this study

Saturated

(for details see Appendix A2). The detailed SOP resulting from this can be accessed as explained in Appendix A6.

The main outcomes for the SMA method testing are:

 The tests should be conducted with 1 g COD l^-1 substrate concentration and 150 ml sludge of medium viscosity (still pourable) resulting in an approximate S/I ratio of 0.05 g COD g VS^-1

 The tests should be conducted with a single substrate addition

 DEWATS-sludge storage times should not exceed one week since storage was clearly shown to have an adverse, and in some cases strongly adverse, effect on the responsiveness and activity of acetoclastic methanogens

 Standard deviation of triplicate measurements was found to be very small with tests done at the Yogyakarta laboratory, especially during the most decisive first 10 h of the experiments.

Results produced there are therefore based on tests conducted with duplicate runs. The SMA investigation results produced by the Bangalore laboratory team are based on triplicate runs since these had considerable standard deviations probably due to leaky pipe connectors.

 Duplicate sequential SMA measurements of samples taken from the same sampling points up to three months apart have a standard deviation of 1% to 12%.

In Indonesia SMA measurements were done in May 2013, at the end of the wet season. They were repeated in the dry season (September 2013) in order to assess whether an extended period without storm-water intrusion would lead to a significant increase of SMA. The last strong rain (120 mm d^-1) however was recorded very late in the year, on June 17^th, about eight weeks before sludge sampling in August 2013. The last rain (10 mm d^-1) even occurred later on July 25^th, or about four weeks before sludge sampling. Assuming that rain does affect the methanogenic population through washout, this was a very short period in which to expect any measurable change. Also, precipitation measurements were done at a 2 km distant site. Local rain occurrences affecting the plant can therefore not be ruled out.