one year of bucher press operational experience at … · converged to a balance tank and treated...

20th European Biosolids & Organic Resources Conference & Exhibition

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ONE YEAR OF BUCHER PRESS OPERATIONAL EXPERIENCE AT OXFORD

SEWAGE TREATMENT WORKS

Macedo, F.1, Fountain, P.1 Huppert, M.2, Pinnow D.2, Webb, I. 1 1 Thames Water, 2 Bucher Unipektin

Corresponding Author Tel. 07747 642475 Email [email protected]

Abstract

After full-scale trial ran at Oxford STW, Thames Water acquired 19 Bucher Press for 4 different STW with the purpose of hydrolysed digested sludge dewatering. This paper covers experience from installation to operation of the presses and through sludge characteristic change (Conventional Digested sludge to Hydrolysed Digested sludge) at Oxford STW. The discussions include learnings on design for dewatering hydrolysed digested sludge, operational settings, performance assessment parameters and performance optimisation, polymer consumption, power consumption and required maintenance from 01/10/14 to 01/10/15. During this period the presses were being operated by Tier 1 contractors responsible to deliver the sludge stream upgrade project and Thames Water had no major accountability for performance.

Keywords

Hydrolysed Digested Sludge, High Dry Solids Dewatering, Operational Experience

Introduction

High Dry solids Dewatering and Wastewater Industry

In Sludge Treatment from the municipal sewage treatment, Thames Water appears to use dewatering technologies for three main applications: a) pre-THP dewatering, in order to get sludge at ideal % Dry Solids (DS) to be fed into Thermo-hydrolysis Processes (THP) reactors; b) cake imports transport between satellite sites and big sludge treatment centres; and c) in the end of sludge treatment, where sludge cake is transport to its final destination.

The technologies historically used for the mentioned above are belt presses, decanter centrifuges and plate presses. Bucher Presses have shown a high capacity to extract liquid in the food market, being also very competitive in the Wastewater Industry. A few presses were then installed in Germany, Switzerland, Sweden and Norway for digested sludge cake production.

Thames Water has run lab-scale trials at its Research & Development Centre followed by full-scale trials in partnership with Kier at Oxford STW. During one year, sewage sludge from existing sites was transported to Oxford to be tested. Table 1 shows a summary of a few key parameters observed during the one year full-scale trial. Anaerobic Digestion or THP + Anaerobic Digestion were the treatment technologies tested, as required by strategic team.

The Bucher Press consists of a large cylinder with a moving piston and parallel filter elements (socks) inside. These socks are made of normal filter cloth with a flexible support structure inside to allow flow through the cloths, down the tubes to the end of the press (e.g. Fountain et al. 2013). Filtrate is extracted from sludge during (1) Filling-pressing and (2) Pressing only cycles. The last cycle of the batch (run) is the discharge, where sludge is pulled out of the cylinder by the pressing piston. The press is ready to recommence the run. What guarantees High Dewatering Solids is the fact that cake is uniformly dewatered during (1) and (2). Cake from belt or plate presses present a skin in contact with the cloth/belt which is much drier, while the cake behind the skin is wetter

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because the water was less able to get away. A schematics of the described mechanism of dewatering is observed in Figure 1. Table 1: Average performance achieved during one year trials with the full-scale

Bucher Press HPS 7507

Parameter Digested Sludge Hydrolysed

Digested Sludge (50% SAS)

High SAS content Hydrolysed Digested

Sludge (80% SAS)

Final Cake %DS 26 to 29% 43 to 45% 38 to 42%

Poly Consumption 8 to 10 kg/TDS 10 to 13 kg/TDS 16 to 18 kg/TDS

Figure 1: Assumed difference in dewatering mechanism between Bucher Press and Belt or Plate Press

Oxford STW sludge stream upgrade project

Oxford STW sludge treatment stream was responsible for treating sludge load of around 280,000 population equivalent (PE) in 2011(load for indigenous sludge plus liquid imports of satellite sites). The project of upgrade of the sludge treatment stream by installation of Thermal Hydrolysis before Mesophilic Anaerobic Digestion (MAD) was approved as a result of population growth in the western area of Thames Water region as much as tighter environmental regulations. As a benefit from the project, it is remarkable the increased potential energy production from biogas out of Thermal Hydrolysis + MAD. This reduces costs from treatment in satellite sites that shall be sending sludge to Oxford as an alternative to liming (average total cost of 100£/TDS).





The original sludge treatment plant load capacity of 24 TDS/d was consequently expanded to a load of 67 TDS/d. The maximum guaranteed capacity load includes a 15% headroom capacity from the expected load of 58 TDS/d of a PE of around 670,000 expected for the area in 2021.

In the new installed facility, sludge is converged to the pre-THP Buffer Tank (2 Silos) via 2 streams: a) cake imports facility (up to 30TDS/d), with rewetting with sludge or final effluent, according to operations choice, and b) dewatered on site indigenous (22 TDS/d) plus liquid imports (10TDS/d). After the silos, sludge is diluted to 16% DS, optimal solids content for THP feed. Liquid Sludge (Indigenous and Imports) is screened on site by Strain Presses (Huber). Cake Imports are screened by each satellite site. The average % Primary Sludge into the Silos is from 40 to 60% with 35 to 45 g of Iron (Dry Weight) per kg DS.

After hydrolysis (Veolia BiothelysTM plant, with capacity to treat up to 70 TDS/d of sludge; 3 pairs of reactors), sludge is diluted to 8-10% DS and pumped to Digesters 1, 2, 3 & 4 (recirculation loop feed). The digester hydraulic retention time target is 13.5 days. Digested sludge spilled out of digesters goes to a first buffer tank by gravity. It is by this point transferred at 90m3/h through a 120 meters gallery to 2 buffer tanks located in front of the existing post-digestion dewatering building. Those two tanks feed the post-digestion dewatering feed tank, which feeds the post-digestion dewatering presses (Bucher Presses). Figure 2 shows treatment flowchart.

Filtrates from pre-THP dewatering belt presses and post-digestion dewatering Bucher Presses are converged to a balance tank and treated in a Cyclic Activated Sludge System (CASSTM) Liquor Treatment Plant (LTP). The biogas storage was also increased, aiming better usage of expected increase in biogas production. Although the LTP and the biogas system are part of the sludge stream upgrade project, they aren’t represented in Figure 2.

Figure 2: Oxford sludge treatment stream upgrade. Assets out of red dot line are part of the upgrade project

Bucher Presses at Oxford Sludge Centre

The post-digestion dewatering technology used before the project was Belt presses. There was a

double interest by the company in acquire new dewatering equipment. Firstly, the existing belt

presses (2x) were in the end of their life time. There was also the need of increasing the available

dewatering capacity. The project purchased and installed four Bucher Presses at Oxford STW.

Polymer storage, make-up and injection, and cake pad were also part of the project.





The presses were sized to cope with full daily throughput of 720 m3 (around 240m3 per press per

day) with only three presses, allowing headroom for maintenance. Sludge throughput can be

pushed to higher than 360 m3/d if required. However, the cake produced would be wetter because

of reduction in pressing time. The most seem run length during the year when data was discussed

is 150 minutes, equivalent to around 10 m3/d.

Methodology

This study is divided in 3 main discussion points: design challenges, operational data and

performance, and conclusion including some financial analysis. All the findings exposed for the

design are part of Thames Water experience of Oxford in comparison with other sites. Operational

data assessed was taken from: a) daily operational collection of flows values in totalizers of the

Human Machine Interface (HMI) or Scada by Tier 1 contractor for the upgrade project; b) week

days sampling of sludge feed, cake and liquors; and c) data acquiring from each of the 4 Bucher

Presses control panel – Bucher Press software (USB).

Data used in the paper was taken from 01/10/2014 to 01/10/2015. During this period, a switch of

sludge feed to the presses from Conventional Digestion to Hydrolysed Conventional was observed.

Digester feed sludge was as per shown in Table 2. The site hasn’t achieved full throughput yet

because of commissioning issues non related to the presses.

Table 2: Digester feed during studied period

Date Digester feed Average ORL Rate

(kgVS/m3/d) Average ammonia dig.

content (mg/L)

01/10/14 to 12/12/14 Raw sludge 2.24 805

13/12/14 to 27/12/14 Raw sludge +

hydrolysed sludge 2.23 1,230

28/12/14 to 01/06/15 Hydrolysed sludge 2.25 2,289

02/06/15 to 01/10/15 Hydrolysed sludge 2.94 2,280

Design - Challenges

The Bucher Presses work in a batch sequence as shown in Figure 3. There are multiple filling &

pressing during phase (1) as piston

moves backwards and forwards

respectively. One fill-press cycle

would typically set up to length 115

seconds, whereas the set up time

for the whole phase (1) was around

60 minutes. Those can change

accordingly to sludge

characteristics. Feed rate should

be taken into account for sludge

feed pump sizing. The P&ID of one

press can be found in the paper

Appendix.

Figure 3: Typical Bucher Press run





Dewatering building

Other Bucher Press installations at Thames Water in STW have the presses allocated in a first

floor, with cake pad in a ground floor. This way, when the Bucher Press cylinder opens for

discharge, the cake drops straight onto the cake pad or truck. At Oxford, the project had to maintain

the existing dewatering building, which required extra ancillaries. During the discharge (3), the

press piston pushes the dewatered cake that drops into a metallic hopper bottomed with 3 cake

cross-conveyors positioned horizontally. These pump cake to a cake inclined conveyor, which

conveys the cake to the external cake pad.

The configuration embraced at Oxford requires 1 hopper, 3 horizontal conveyors and 1 inclined

conveyor for each press. The cost of the mentioned above, level sensors and metallic structure

increased the price of the installation. For this choice of design, enough space needs to be provided

for maintenance of screws and Hydraulic parts of the Presses. The configuration also requires extra

daily housekeeping maintenance for the extra conveyors.

Polymer make up and injection

The polymer make-up facility holds a powder polymer silo sized to keep 35 tons of powder polymer.

An airflow pumps polymer to a pre-dilution tank, which is done with potable water. There is a second

dilution with Final Effluent into a second tank. The polymer concentration target of the second

dilution is 0.3 % w/w.

The polymer dosage target is as shown in Table 1 (10 to 13 kg/TDS). This parameter is set up in

each press control screen. To deliver the required dosage, each press assumes the concentration

the operator inserts for polymer dilution (%) and the %DS of sludge measured (ABB meter) or set

up in the press screen. %DS measurements were taken to guarantee reliability of %DS meter in

sludge feed. Each press has its own

polymer, sludge and mixed line as in

Figure 4.

Because the presses were installed at

3.00 m from the ground, a mixer was

installed where polymer is injected. This is

to avoid polymer to stay in the lowest point

of the pipeline, as observed by Thames

Water Innovation Team during the full-

scale trial. Such issue had avoided good

flocculation. The pump has been set up to

run at 960 rpm and it works as a high-

energy mixer device, helping polymer to

be mixed to sludge. Figure 4 shows the

described above.

Daily operational tasks were required in

the past year in order to check if

flocculation was sufficient (visual check of

mix flocs and liquor quality).

Discharge and Cake Pad

The cake pad at Oxford has a roof to avoid rain of rewetting cake. Tractors can freely manoeuvre

in the cake pad to fill cake trucks. The cake piles go from 3.5 to 4.5 m high, with very good stability.

The cake pad is well ventilated which make ammonia control system unnecessary. It has been

observed in other sites (indoor cake pad) high ammonia stripping from the cake and whitish layer

of Fungi in the surface of the cake, which may be an indication of composting of the material. There

were no concerns about such between farmers.

Figure 4: Polymer injection into sludge feed line





%DS of cake produced depend on run length and will be discussed in performance items below.

Filtrate

The filtrate is squeezed out of the cylinder during phases (1) and (2) of Figure 3. It comes out of

the press pressurised and failures due to “filtrate line busy” were spotted in other sites. Separate

project teams in Thames Water sites dealt with the issue with different solutions. At Oxford, the

common filtrate line to the 4 presses has a big diameter to avoid the issue. Filtrate goes to Liquor

Balance Tank by gravity, from where it is pumped to required treatment mixed with filtrate from pre-

THP dewatering.

Data of filtrate ammonia and SST (mg/L) can be observed in Figure 5. There is no operational data

for ammonia in the period where digester feed was raw sludge only. The ammonia concentration

in the liquors follow ammonia concentration in the digested sludge (Table 1), which is lower than

experienced in other Thames Water THP sites (can be as high as 3,500 mg/L). This may be

because of a low digester feed. High SST samples are related to events where cloths of filter

elements pulled out of the filtrate plate plastic holder. A daily visual inspection is required to

guarantee that all filter elements are perfectly operational. It is recommended that a damaged filter

element should be changed as soon as spotted by the operator.

Figure 5: Bucher Presses liquors ammonia and solids concentration at Oxford STW

Operational data and performance

Bucher presses feed and hours run

Figure 6 illustrates the sludge feed to Digesters and to the Bucher Presses during the period studied

in m3. There is a discrepancy between digester feed and presses feed. This can be explained by

buffer tanks in the stream and potential issues during commissioning that would have backlogged

sludge. The red trend and the green trend represent respectively presses feed acquired daily by

operators in the HMI and presses feed acquired in each press control panel (SIEMENS). In the

majority of the time, the green line is delayed one day. Because of discrepancy between those, this

study will use data acquired from each press rather than data daily acquired by operators.





Figure 6: Digester feed and Bucher Press feed volume in m3

Figure 7 is a register of total hours run per press. This survey is important for maintenance

purposes. All filter elements were changed in march/2015. The recommended by Bucher Press is

that the filter elements of a press could require change when the hours run reach 2,000 to 2,500

hours, or performance decreases. The change depends on sludge characteristics. At Oxford, no

decrease in performance was been observed that justified a change, but the project team decided

to change it to align with other part of the project programme. The green double arrow indicates

the period when the sock changes happened. The red dot line shows a diagonal that a press would

follow in case it ran 24 hours every day. It shows that it hasn’t happen because of lower than design

digester feed. Press 1 was the Innovation Thames Water full-scale trial, which explains its higher

hours run in the beginning.

Figure 7: Bucher Presses hours run across one year of operation

Sludge feed %DS and cake %DS

The Presses rely on readings from DS meter between the feed tank and the presses to determine

polymer usage and to estimate cake %DS. Figure 8 shows %DS of inlet sludge. There is god

accuracy between sludge %DS sampled and %DS read by meter. The project team sampled





sludge daily and worked with correction factors when required. Figure 9 shows %DS from samples

collected in presses feed tank and from each of the presses during discharge phase. Both graphs

have 4 dot lines that indicate the following:

1st) Black line (11/12/14): date from when sludge feed into digesters was both raw and hydrolysed

2nd) Red line (22/12/14): date from when sludge feed into digesters was hydrolysed only

3rd) Green line (18/02/15): observed change in product cake %DS

4th) Blue line (26/07/15): observed change in cake %DS of press 4 due to length of run change.

There is an increase of %DS feed from the beginning of July/15 that may be an effect of digester

Organic Load increase followed by an inconsistent digester feed (Table 2 and Figure 6).

Regarding %DS of cake (Figure 9), it can be observed that it drastically increased at the green line.

This date is 68 days after feed started to change to hydrolysed and 57 days after feed has been

completely changed to hydrolysed sludge. Digester retention time during this period averaged

around 32 days (14 to 50 days). For this site, it is observed that sludge took a period of 2 retention

times to obtain dewaterability expected from hydrolysed sludge. However, it is important to highlight

that digester feed was inconsistent during the period and at a lower feed than expected for THP +

MAD.

There is a drop in performance of press 4 (blue line). This is because of change of run length in

this press to keep throughput during issues with ancillaries faced by the project team in presses 1

and 2.

Figure 8: %DS inlet sludge. Blue dots are readings from sludge sampled and purple

dots are readings taken from ABB %DS meter in presses software





Figure 9: %DS sludge inlet and cake from all presses from samples

Polymer consumption, run time and total sludge fed per run

Data in this session was acquired at each press and help evaluating performance. Values of each

run are represented by a dot, reason why vertical lines of dots are observed for a given date.

Figure 10 shows polymer consumption during the period. This is a parameter changed by operator

in each press control panel. The project team chose to start the presses with higher consumption,

and as they got confidence in the presses, polymer consumption was reduced and stayed below

the company’s target (Table 1) set up after full scale trial (green area). The site is used the same

powder polymer diluted to 0.3% w/w during the whole period. Good %DS cake was achieved with

11 kg/TDS. Once the polymer consumption was been set up, the presses genuinely delivered the

required set point (observed overlapped points for a given date and constancy over a period).

Figure 10: Polymer consumption in kg of Polymer/TDS of sludge – set up by operator

and dependent on %DS feed meter. Green area represents current target

Figure 11 is reporting time taken in a run to dewater sludge (length of batch). The length of a run

can be determined by the operator based on different requirement as length of run only or minimum

cake %DS required. Hence, pressing only cycle time rises to achieve required %DS. This may





explain a few dots observed above the 150 minutes. Other reasons may be related to ancillaries’

issues. As mentioned above, the length of run was changed in July to suit project requirements and

can explain drop in performance observed in press 4 in Figure 9.

Figure 11: Run time – parameter set up by operator

Figure 12 shows the amount of sludge fed by run. This is a performance parameter that helps

understanding if there was a good reaction between sludge and polymer and if flocculation and

flocs size were suitable for the process. It is therefore related to sludge dewaterability and can

explain either mechanical issues with polymer make up and/or pumping or sludge biological/

chemical characteristics change. It happens because the moving piston is set up to achieve a

certain distance to get expected dewatering during each sequence of filling and spilling phase. If

the piston can’t go beyond this set point, it means that free water in the mix (flocculation) isn’t ideal

for pressing. The press will try to push piston beyond the set point again without a new fill, which

reduces volume fed per run. A more optimal flocculation leads to better drainage of water. This

allows higher filling volumes and higher cake %DS (lower cake volume).

Figure 12: Sludge fed per run - shows sludge dewaterability





Before the period studied, the presses were running at a high volume fed per run. There were a

few issues during the transition between raw sludge and hydrolysed sludge feed (second red line

to green line). From the green line (18/02/15), sludge volume fed went up to above 25,000 L, which

is considered optimum for the year.

There was a reduction of sludge fed in the period after length of time change (from the blue line -

26/07/15). However, this change isn’t as significant because the team selected reduction from

“pressing only” cycle, rather than filling. The result is lower cake %DS and increased flow from 8-

10 m3/h to 12-14 m3/h.

Figure 13 also evaluates performance and the results observed should be analysed in combination

with Figure 11. The parameter kg/h can be used to predict expected cake %DS for each type of

sludge.

Figure 13: Performance measured in kg of sludge fed per hour

Maintenance and housekeeping The project team has given positive overall feedback of housekeeping and maintenance around the Bucher Presses. Time spend around the presses when their ancillaries were properly working went from 1-2 hours per day. This includes visual checks of filter elements, grease levels, noise assessment, filtrate visual check, sludge feed and cake %DS sampling and reading in moisture analysers. Most of issues that the commissioning team had to deal with were related to conveyors maintenance and poor water supply installation. However, special caution and care is required with filter elements and hydraulic grease suppliers. It is recommended detailed inspection advised by Bucher Unipektin in filter elements deliver and hydraulic grease to be used.

Conclusion

The paper summarised some considerations taken during design phase and was extremely focused in performance date acquired during the first operational year (under commissioning). It helped understanding how different parameters can be used to evaluate performance and understand potential issues with the dewatering unit (ancillaries or presses). The average TDS throughput during the period was 10.1 TDS/d against 28 TDS/d expected for full throughput. The lower throughput is related to problems in other units of the STW. The digesters haven’t got the expected full throughput neither.





It has been proved that the presses met site requirements during the period. Bucher Presses average %DS of cake produced from hydrolysed digested sludge was 38-42% during this first year of commissioning – period where presses haven’t been taken over by Thames Water yet. This performance was obtained with polymer consumption between 10 to 12 kg/TDS, according to operator set up.

References

Fountain, P. Lee, K., Kellari, S., and Webb, I.(2013) High Dry Solids dewatering – from pilot scale through to full scale implementation comparing performance. 18th European Biosolids Conference, Manchester, Nov/2013 Bucher Unipektin. HPS/X 207, HPS/X 3007, HPS/X 6007, HPS/X 7507, HPS/X 12007 – Operating Instructions Commissioning team (2014-2015) Commissioning Log Book Thames Water Utilities (2014-2015) Oxford STW Operational Log Book for post-digestion dewatering Bucher Presses HPS/X 7507 data at Oxford STW (2014-2015)





Appendix


one year of bucher press operational experience at … · converged to a balance tank and treated...

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