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20th European Biosolids & Organic Resources Conference & Exhibition

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EAST LONDON THP COMMISSIONING AND LABORATORY INVESTIGATIONS

Aurelien Perrault1, Ester Rus1, Paul Fountain1, Achame Shana1

1 Thames Water, Reading, UK

Corresponding Authors Tel: 07747647208 email: [email protected]

07747640614 email: [email protected]

Abstract

THP Digestion plants at Crossness and Beckton are in their final stages of commissioning. The decision was made to run a set of chemostats in parallel with the full scale sites in order to better understand the digestion and dewatering of the sludge. The work was carried out by Thames Water Innovation.

A bench scale trial consisting of 10L chemostat digesters which were set up to compare the site stability parameters and dewatering with a controlled environment. The addition of iron sulphate into the chemostats showed a clear improvement in dewaterability of the digested sludge with increasing doses of iron.

Keywords

THP, Dewatering

1. Introduction Crossness and Beckton STWs are Thames Water’s two largest sites with a total of about 5.5millions PE. Until 2014, the sludge produced on those sites was mainly processed through the Sludge Powered Generators implemented on both sites after the sludge to sea disposal ban in 1998. The two SPGs allowed for significant sludge volumes reduction through combustion of raw cake but also for some energy recovery from sludge through heat recovery and electricity generation (steam turbines). Based on the recognised success of Thermal Hydrolysis Process (THP) as sludge pre-treatment to Anaerobic digestion on various sites across the UK (e.g. Chertsey, Cotton Valley, Riverside, Cardiff), Thames Water decided to implement this technology on 5 other major sites, including Beckton and Crossness STWs. Due to the lower overall OpEx costs and higher electrical generation per TDS of the THP process compared to the SPGs, the strategy on these sites was to maximise the throughput of the THP plants but keeping the SPGs operational in order to treat the remaining sludge. The commissioning of Crossness and Beckton started respectively in the summer and autumn of 2014. 2. Methodology The set of six laboratory or bench scale semi-continuous anaerobic digestion (AD) rig consists of six 10L spherical glass chemostats with 8L working volume (WV), each with overhead stirrers for continuous mixing, aspirators with graduated biogas collection bottles, and a common water bath (see figure 1).

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Figure 1: (a) schematic of semi-continuous AD rig and (b) picture of chemostat in a

waterbath (Shana, 2015).

Chemostats 1, 2, and 3 were seeded with digested sludge from Beckton and chemostats 4, 5, and 6

with digested sludge from Crossness. Ferric sulphate was also dosed at different % by weight (see

table 1):

Table 1: Sludge precedence, OLR and Fe dose for each chemostat:

Sludge samples were taken from site each day – after blending, after pre-THP dewatering, after THP,

after post THP dilution – the exact same feed that was going to digestion, as it was going into the

digester recirculation lines.

Every day, hydrolysed sludge from both sites was imported in 1L bottles to TW Innovation facilities to

feed the chemostats. Each chemostat was fed once a day according to the designated organic loading

rate.

Volatile fatty acids (VFA), alkalinity, pH, ammonia, and gas production were checked 3 times a week.

Furthermore, dewatering trials with a bench scale piston press operated on a protocol which would

mimic belt press performance. More recently a standardised Bucher Sock test has been developed by

Bucher which is able to measure the polymer consumption and the cake dry solids that would be

achieved by a full scale press. Chemical analysis was done every 2 to 3 weeks on both chemostats and

site sludge, including total and soluble phosphorous, magnesium, calcium, potassium, and sulphate.

Sludge was also analysed for iron content.

Chemostat 1 2 3 4 5 6

Site Beckton Beckton Beckton Crossness Crossness Crossness

OLR (kgVS/m3/d)

3 3 3 3 3 3

Fe dose (%w/w)

- 0.2 2 - 0.2 2




3. Results and discussion:

3.1. Stability of Chemostats:

Table 2 shows the stability parameters monitored:

Table 2: Stability parameters: Alkalinity, VFA, pH, Ammonia, methane content in biogas.

Chemostat Alkalinity (mg/L)

VFA (mg/L) pH (-) Ammonia (mg/L)

CH4 (%)

1 - Beckton 9,849±214 769±69 8.01±0.09 2,558±231 67.4±5.5

2 - Beckton 9,609±177 758±51 8.01±0.08 2,763±358 67.5±5.5

3 - Beckton 9,420±241 754±49 8.03±0.11 2,653±413 66.7±4.4

4 - Crossness 9,494±552 750±73 7.99±0.08 2,534±248 66.7±5.8

5 - Crossness 8,919±210 682±53 7.98±0.08 2,458±299 66.6±4.3

6 - Crossness 8,734±330 630±57 7.94±0.11 2,878±330 66.7±4.7

Some variability on the alkalinity and VFA values was observed due to instrumentation issues.

Nevertheless, both parameters remained stable for most of the recorded period. Post THP dilution was

controlled on site to maintain the digester ammonia levels below 3,000mg/L. The methane content in

the biogas remained between 66-67% for all chemostats.

3.2. Complete Hydrolysis – gas generation and volatile solids destruction:

Both sites have Mark 1 CAMBI reactors, with a 30 min cycle at 6barg pressure and 165°C temperature

followed by a venting cycle of circa 10 min to drop pressure slowly from 6 to 3barg before flashing the

hydrolysed sludge from the reactor into the flash tank, with a sudden pressure drop from 3barg to

atmospheric. The good gas production and volatile solids destruction (VSD) achieved averaging

420m3/tDS and 56% respectively at both Crossness and Beckton STWs where within the expected

range for mesophilic anaerobic digestion (MAD) with thermal hydrolysis pre-treatment.

The chemostats were fed with hydrolysed sludge from site based on that assumption and the results

were similar to those seen onsite:

a. Gas generation:

The performance of all six chemostats matches what is expected and specified by the asset standards

for a MAD with THP pre-treatment. Figure 2 shows the biogas production per kg of volatile solids

destroyed (VSD):




Figure 2: Chemostat Biogas production (m3/kg VSD) and OLR (kg VS/m3/d).

After a period of ramp up, all 6 chemostats reached stability and achieved an average of 1 m3/kgVSD

under stable conditions.

Figure 3: Chemostat Biogas production (m3/tDS) and OLR (kg VS/m3/d).

Figure 3 shows the biogas generation in m3/tDS. As can be seen, all chemostats are producing an

average of 420m3/tDS, which is within the expected range for THP + MAD.

b. Volatile solids destruction:




The VSD achieved in the chemostats also lies within the expected 50-55% for all 6 chemostats. Figure

4 shows the VSD of all chemostats calculated with (a) the mass balance (MB) method and (b) the Van

Kleeck (VK) method.

Figure 4: Chemostat VSD (a) Mass Balance and (b) Van Kleeck.

3.3. Effects of iron dosing on chemostats:

It has been found that the effluent streams at both sites were opportunistically operating in BNR

(Biological Nutrient Removal) mode – not by intention, or design, but none the less achieving biological

absorption of soluble phosphorous from the effluent stream into the sludge. While this reduces the

amount of soluble phosphorous in the effluent stream, when that sludge gets into the digesters – in an

anaerobic environment with carbon source – it then releases that same soluble phosphorous into the

digesters. While soluble phosphorous itself is not a problem, it reacts with soluble magnesium and

soluble potassium which are very much needed in the polymer, flocculation and dewatering process

post digestion.

A lot of research has been done on the effect of iron dosing on soluble phosphate and monovalent to

divalent (M/D) ion ratio. Higgins and Novak (1997b) concluded that the M/D ratio was positively

correlated with the specific resistance of sludge to filtration, and therefore, cakes with higher DS could

be achieved with sludges showing lower M/D ratios. This was also found by Alm et al., (2015) and Park

(2002). Higgins et al., (2014) suggested that biological phosphorous is key to understanding

dewaterability issues in digested sludge. According to this research, high concentrations of

phosphorous in the digester can result in a decrease in available magnesium and potassium ions, which

play an important role in biofloc formation prior to digested sludge dewatering.

For this project, a controlled experiment was set up with sludge from Crossness and Beckton STWs to

test this theory. Digested sludge from both sites was analysed for soluble ion concentrations under six




different iron doses with ferric sulphate: 0% (no dose), 0.1% (w/w), 0.2% (w/w), 2% (w/w), 5% (w/w),

and 10% (w/w). This wide range of iron doses was useful to understand the relationship between iron

dose, M/D ratio, soluble phosphorous and dewatering performance. Figure 5 (a) and (b) show the

relationship of increasing ferric content in digested sludge with the concentration of soluble

phosphorous and the M/D ratio for Crossness and Beckton sludges respectively.

Figure 5: Relationship of Sol P and M/D ratio with increasing iron concentrations for (a)

Crossness and (b) Beckton digested sludge.

Both soluble phosphorous concentration and M/D ratio decreased with higher iron content in the

sludge as expected, which lead to an increase in DS in the cake, as shown in figure 6:




Figure 6: Cake DS (%) with increasing iron doses in digested sludge from (a) Crossness

and (b) Beckton STWs.

The iron doses chosen for the chemostats where more conservative with no dose, 0.2% (w/w) and

2% (w/w) for each site.

Figure 7 shows the three chemostats from each site under the different treatments. The increase in iron

content in the lab scale digesters did have an impact on the concentration of soluble phosphorous but

not on magnesium or calcium. Higgins et al., (2014) saw higher concentrations of both magnesium and

calcium ions in sludges with lower soluble phosphate. Nevertheless, the changes in iron content

observed in the chemostats may be too small to significantly shift the M/D ratio.




Figure 7: Soluble ions (mg/L) and iron content (g/kg) in (a) Beckton and (b) Crossness

chemostats.

The dewatering trials (or phases) done on the chemostats under the different iron doses did show a

significant increase in DS with increasing ferric sulphate. See figure 8:

Figure 8: Dry solids in piston press cake with different Fe doses (% w/w). Dewatering

with bench scale piston press.




Note: These trials were done using a belt press protocol and hence the target for good

performance was 30% dry solids.

Overall, the effect of iron dosing on the chemostats had a positive impact in the dewaterability of

digested sludge. One of the challenges has been that there was still a difference between dewatering

of sludge coming from the digesters at both sites and the chemostats. In all cases, digested sludge from

the chemostats under similar iron conditions as those seen onsite formed a better floc with significantly

less polymer doses required, achieving higher DS in the cake.

3.4. Effect of reseeding chemostats 3 (Beckton) and 6 (Crossness):

To understand whether the conditions of digestion onsite compared to those in the chemostat were

having an impact on dewaterability, two chemostats (Chemostat 3 from Beckton and Chemostat 6 from

Crossness) were reseeded and dewatering tests with the piston press where done twice a week for a

period of 4 weeks. Both DS and polymer dose were recorded. The results can be seen in figure 9:

Figure 9: Reseeding of chemostats (a) 3 with digested sludge from Beckton STW and (b)

6 with digested sludge from Crossness STW and monitoring of dewaterability

and polymer demand.

Note: These trials were done using a belt press protocol and hence the target for good

performance was 30% dry solids.




The iron dose remained constant at 0.2% (w/w) to match what was then dosed onsite. Therefore, under

same iron conditions, a clear improvement in dewatering as well as a gradual decrease in polymer

demand was observed in both cases at lab scale. This would indicate that something else apart from

the iron addition is affecting the dewatering performance.

Investigations are still under way to confirm what the additional factor might be.

4. Conclusions

The work done to support the THP commissioning phase at Crossness and Beckton STWs showed

that the presence of soluble phosphorous may partly be causing a worsening on the dewatering of

digested sludge by altering the cation balance during digestion. Iron dosing did show benefits in

dewatering. Nevertheless, the bench scale digesters or chemostats did show better dewaterability

and lower polymer demand than site under the same iron dose conditions. Therefore, something else

is believed to be contributing to the dewatering issues. Investigations are still under way to confirm

what this might be.

5. References

Alm, R., Sealock, A.W., Koo, A., and Sprouse, G. (2015) Investigations intro improving dewaterability

at a Bio-P/anaerobic digestion plant. WEF/IWA Conference. June 2015, Washington DC.

Higgins, M.J. and Novak, J.T. (1997b) The Effect of Cations on the Settling and Dewatering of Activated Sludges: Laboratory Results. Water Environ. Res., 69, 215. Higgins, M., Bott, C., Schauer, P.(2014) Does Bio-P Impact Dewatering after Anaerobic Digestion? Yes, and not in a good way! WEF Residuals and Biosolids Conference. Park (2002) Cations and activated sludge floc structure. MSc Thesis. Virginial Polytechnic Institute

and State University.

Shana (2015) Application of an Innovative Process for Improving Mesophilic Anaerobic Digestion of

Sewage Sludge. PhD Thesis. University of Surrey.


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