3 4 5 Compartments

down the reactor since acidogenesisis the only phase of anaerobicdigestionoccurring in the reactor.

40000

i ^{30000 20000}

~ ~ 10000

0

1 2

.TSS

3 4 5 Compartments

6

ElV S S

7 8

Figure 4-17: Characterisationof solids for each compartment on Day 50 of operation in 2003

Chapter4 The Pilot Scale ABR

Characterisation of solids for each compartment showed that there was increasing biomass down the reactorwith the except ion of compartment 6 (Figure 4-17).Compartment 6 has beenthe exceptionina number of results (Figure4-15 and 4-11).The results also showthat the solidsin compartment 1 and2 although occupying a largevolume of the compartments were light because they had lowTSS and VSS.

This was indicativeof the presence of the fatty scum in the compartments 1 and 2.

Table 4-4: Ratios of TSS to VSS for each compartment

Compartment Ratio

1 3.1

2 3.0

3 3.3

4 3.3

5 3.0

6 1.3

7 2.4

8 2.2

Table4-4 shows that the ratio of TSS: VSS is ca. 3 forcompartment 1 to,5,for compartment 6 the ratio decreasesto 1.3,and increases to above 2 for compartment7 and 8. The amount of VSS in compartment 6 is close to that of TSS suggesting the compartment consists mainly of biomass, and this happens in conditionsof excesssubstrate (section2.2.2.).This linksin with the results in section4.5.3.,where soluble CODwas present throughout the reactor.

1200 90

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1000

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0 10 20 30 40 50 60 70 80 90 100

Day of operation (2003) Figure4-18: Total suspended solids in and out ofthe ABR

Figure4-18 shows totalsuspended solids concentrationsof the influent and effluentstreams ofthe ABR.

All effluent TSS values were below 400 mg/L with an average of225 ± 96 mg/L. The removal curve follows the influent curve,because the amount of solids coming out of the ABR does not varywiththe influent.On Day56 the influent TSS decreased with the dilutionof storm water but thisdid not decrease the amount of TSS in the effluent. This suggests that the amount of TSS in the effluent is aboveall related to the liquid upflow velocitywithin the reactor.The average TSS removal was ca. 50%.

Chapter 4 The Pilot Scale ABR

1

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^{10 20 30 40 50 60 70}

Day of operation (2003)

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Figure4-19:Volatile suspended solids in and out ofthe ABR

Figure4-19shows the volatile suspended solids in the effluent like TSS were they were not dependent on the amount coming into the reactor.The average VSS of the effluent was ca. 100mg/L and the removal was about 60%.

4.5.5

Discussion

The number of incidents was reduced for the 2003 trial period. With the aid of a PLC the feeding to the reactor was kept constant and an average HRT of 22.5 h was achieved. This allowed the solids levels increase to about twice the amount obtained in 2002.The alkalinity in the effluent was greater than that in the influent indicating low production of carbon dioxide which is produced in acetotrophic methanogenesis, indicating a low rate of methanogenesis.The 100mg/L of soluble COD leaving the reactor confirmed the incomplete degradation of COD in the reactor. The pH inside was stable and almost constant from compartment 2 to discharge at low values that prevented methanogenesis.The actual pH profile within the reactor depended only the on the pH of the feed but the pattern was always the same.The amounts of TSS and VSS in the effluent were independent of the amount in the influent suggesting that the amount solids in the effluent depended mainly on the upflow velocity within the reactor. The ratio ofTSS:VSS is ca. 3 for compartment 1 to 5, for compartment 6 the ratio decreases to 1.3,and increases to above 2 for compartment 7 and 8. The amount ofVSS in compartment 6 is close to that ofTSS suggesting the compartment consists mainly of biomass,and this happens in conditions of excess substrate

c.r.

2.2.2.The reactor is operating as a single phase digesterperforming primarilyacidogenesis because methanogenesis was not occurring.

Chapter 4

4.6. Analytical cam paign

The Pilot Scale ABR

An analytical campaign was undertaken to obtain hourly variations in ABR influent and effluent characteristics. The campaign was carried outon the 29^thof May,a week after the reactor trial period.The reactor was kept running while preparing for the campaign. Hourly samples were taken for 24 h on the influent,and after 20 h (roughly one retention time) hourly samples of the effluent were take for a further 24 h in order that corresponding profiles could be obtained. pH and 0.45 urn filtered COD analyses were performed during the campaign. Due to the large number of samples taken, the measurement was performed in duplicate only by the closed reflux method so that samples could be analysed before significant changes in composition could occur. Samples were homogenised using a hand-held blender to reduce random sampling error before acidification and storing below 4 °C.

1500 1250 1000 750 500 250

o

9:00 12:00 15:00 18:00 21:00 0:00

Time

3:00 6:00 9:00

Figure 4-20: Influent COD, effluent and filtered effluent COD profiles translated back by 20 h

The effluent curve has been transposed 20 h back in time so that effluent characteristics can be directly compared with influent characteristics.It should have been transposed back 22 h,but the calculationof the retention time at the time was done after the campaign. The influent COD profile shows an increase in COD of the wastewater coming into the plant between 10:00 and 14:00.There is no corresponding peak in the effluent COD profile indicatingthe reactor is able to digest the peak.The influent COD profile shows a cyclical trend with period of 1 d where the COD starts to increase at 06:00 going to 10:00. The effluent COD concentration~ason average under 200 mgIL,basically remaining the same compared to the routine analysis results. The filtered effluent COD concentration were constant at 64 ± 6 mg COD/L, this has decreased from 82 mgIL from the routine analysis results. The slight improvement could be due possibly to better environmental conditions on the day such as temperature. The 02:00whole effluent sample had a

Chapter 4 The Pilot Scale ABR

performance and is attributed to an overflow of anaerobic sludge from the last compartment as a result of gas release or fluid dynamics in the last compartment.

The average COD obtained from the routine analysis was 721 mg/L, from the campaign the average daily influent COD is about 564 mg/L. The influent daily variation shows that the routine analysis sampling time (09:00 to 12:00) coincided with the daily influent COD peak. The average daily effluent COD from routine analysis results was 198 mg/L, but from the campaign it was 234 mg/L, it was increased by the biomass washout which occurs as a peak or slug outside the routine analysis sampling time. If COD removal is calculated from the campaign data a value of 42% is obtained as opposed to 71% from using routine analysis results.

8.0

7.5

i

^7.0

6.5

6.0

9:00 12:00 15:00 18:00 21:00 0:00