Monitoring results: sludge composition, build-up and activity

6.3.5.1. Climatic factor precipitation

Figure 68 illustrates when sludge investigations took place during Phase II and relates them to the precipitation measured during that period. Rain was not measured during Phase II before April 2013 but significant precipitation at that time of the year would be very unusual. On the other hand strong rainfall is probable on the days in August for which no precipitation measurements are available since this is the wettest period of the year. Based on the precipitation data, Phase II was divided into “Phase II - Dry season” (April to June 2013) and “Phase II - Wet season” (July to October 2013).

The available precipitation data for Phase I was too incomplete to be further interpreted.

Figure 68: Precipitation data and sludge sampling and height measurement dates in Phase II

6.3.5.2. Sludge heights

Figure 69 a and b show a selection of available sludge height data in each ABR chamber after the start of operation in 2010. (Showing all available data-points would confuse the chart. A compilation of all sludge height data can be accessed as explained in Appendix A6) Most data points represent the average of duplicate measurements. The ABR chambers were never desludged.

During Phase I sludge increased continuously in each chamber over time. The sludge level was always highest in the first chamber and constantly decreased towards the rear.

During Phase II however the sludge heights in the first two chambers rather decreased over time after reaching a certain maximum of 50 cm to 80 cm. The sludge levels in ABR 4, ABR 5, ABR 6 and ABR 7 on the other hand constantly increased until September 2013. In October all levels measured in ABR 5 to ABR 7 decreased dramatically whereas an equally strong increase occurred in the following reactors.

In October field staff also noticed washed out sludge in the channel behind the ABR. This strong sludge migration and washout clearly correlates with the high intensity rainfall recorded in late October (see Figure 68).

During Phase II the highest sludge levels were found in the middle chambers.

Period without precipitation measurements

Dry season Wet season

0 10 20 30 40 50

01.04.13 01.05.13 01.06.13 01.07.13 01.08.13 01.09.13 01.10.13

Precipitation (mm d-1)

Sludge height measurement Sampling sludge for TS/ VS Sampling sludge for TS/ VS & SMA Sampling COD

Figure 69 a and b: Selection of settled sludge levels measured in Beedi Workers Colony, Phase I and II

6.3.5.3. Sludge volume increase

Figure 70 shows the increase of total ABR sludge volume over the time of operation. Linear regression of the Phase I values indicated an approximate sludge increase of 2 l d^-1 or 0.7 m³ y^-1 in the chambers ABR 1 to ABR 5. The amount of sludge washed into the rear chambers over that period was minimal.

The sludge build-up in the chambers ABR 1 to ABR 5 during “Phase II – Dry season” (based on the measurements taken from 04.01.2013 to 17.06.2013) was estimated at 2.1 m³ y^-1 with about 0.4 m³ y

-1 accumulating in the chambers downstream.

During “Phase II – Wet season” large amounts of sludge started to migrate from the first 5 chambers to the chambers beyond. The sludge build-up in all 12 ABR chambers in this period (based on the measurements carried out from 17.06.2013 to 16.09.2013) was approximately 11.7 m³ y^-1. The last sludge height investigation on October 21^st, directly after the strong rainfall presented in Figure 68 indicated further strong washout from the first 5 chambers and a general sludge volume reduction in the complete ABR.

Figure 70: Sludge volume evolution in Beedi Workers Colony ABR chambers

6.3.5.4. Sludge Total and Volatile Solids concentrations

No sludge TS and VS concentration data was available for Phase I. The composition of settled sludge was measured for reactor chambers ABR 1 to ABR 6 in 2013 as part of the SMA investigations during Phase II. The results are shown in Figure 71 a and b. A tendency can be observed of the TS and VS concentration being highest in the first two ABR chambers and constant or lower in the following

Settled sludge volume (m³) Sludge volume in all 12 chambers in Phase II

Sludge volume in first 5 chambers Sludge volume in last 7 chambers

a) b)

approximately 50 g l^-1 and 26 g l^-1 respectively with a respective standard deviation of 30% and 29%.

The sludge concentrations measured directly after the strong rain at the end of October 2013 suggest a shift: most dense sludge was no longer found in the first two but rather in the rear chambers.

Figure 71 a and b: Settled sludge TS and VS average concentration profiles, number of measurements in brackets, error-bars indicate standard deviations of multiple measurements

6.3.5.5. Specific methanogenic activity (SMA) of sludge

Two SMA measurement runs were conducted for this study. Both sludge sets were sampled during the

“Phase II – Wet season” period, one before and one right after the strong rainfall recorded on October 20^th (see Figure 68).

The SMA results with sludge sampled on September 30^th indicated an uneven distribution between the chambers with highest SMAmax values measured in ABR 2 and 4 of approximately 0.1 g COD g VS^-1 d

-1. The second run yielded a SMAmax value of approximately 0.15 g COD g VS^-1 d^-1 in the first and steady decrease in the consecutive chambers.

The results suggest a strong increase in chamber 1 and 3 and a decrease in chamber 4 and 6. The SMAmax values were comparably similar in the chambers 2 and 5. In general the highest acetoclastic methanogenic activity was observed in the first chambers.

The observed high variability of measurement results is difficult to interpret since the SMA measurement was a comparably new method for the laboratory team in Bangalore and these were the first successful measurement runs produced by that team. The effect of storm water intrusion on the SMAmax value can therefore not be ascertained but further measurement runs are needed in order to approximate the variation of measurement results under undisturbed operational conditions.¹³ It is however conceivable that the strong rainfall recorded on October 20^th washed active sludge from the digester into the first ABR chambers, therefore increasing the SMA.

13 Sequential SMAmax investigations of the same sludge performed in the Yogyakarta laboratory had shown a maximal variation of 12% over 2 months of operation (see Section 3.4.6)

0

Figure 72: SMAmax values of sludge sampled from different reactors in 2013 at BWC, all sludges were processed within one week after sampling, all values were derived from single measurements