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  • 20th European Biosolids & Organic Resources Conference & Exhibition

    www.european-biosolids.com Organised by Aqua Enviro


    Aurelien Perrault1, Ester Rus1, Paul Fountain1, Achame Shana1

    1 Thames Water, Reading, UK

    Corresponding Authors Tel: 07747647208 email: [email protected]

    07747640614 email: [email protected]


    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.


    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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  • 20th European Biosolids & Organic Resources Conference & Exhibition

    www.european-biosolids.com Organised by Aqua Enviro

    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


    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


  • 20th European Biosolids & Organic Resources Conference & Exhibition

    www.european-biosolids.com Organised by Aqua Enviro

    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):


  • 20th European Biosolids & Organic Resources Conference & Exhibition

    www.european-biosolids.com Organised by Aqua Enviro

    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:


  • 20th European Biosolids & Organic Resources Conference & Exhibition

    www.european-biosolids.com Organised by Aqua Enviro

    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