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European Biosolids and Organic Resources Conference 15-16 November, Edinburgh, Scotland

CONTROLLED SLUDGE STREAM: A SMART WAY TO OPTIMISE YOUR ASSETS

Masse, A-L.1

1Thames Water, UKCorresponding Author Email anne-laure [email protected]

Abstract

Digestion performances are impacted by the quality and the stability of the feed to the digesters. At Reading sewage treatment works STW (210,000PE), unthickend sludge is back mixed in the thickened sludge pump located at the end of the belt thickeners to reach the target dry solid (DS) set by the operator. Monitoring has shown that in this configuration torque of the thickened sludge pump and pressure in the pipework were better parameters to use to control the system than DS meter. At Swindon STW (230,000 PE) a similar system to control the sludge stream was designed.

The run time of the primary settlement tanks desludging pumps is based on the thickness of the sludge extracted. This can produce a sludge of consistent DS, allowing the downstream sludge thickening process to run more consistently. To optimise the polymer use and decrease the operational costs, the polymer dosing rate to the thickeners is based on the flow to the thickening unit, on the DS of the influent and on the total suspended solids of the filtrate as this value changes more rapidly than the sludge DS, allowing a more efficient control. To stabilise the DS of the sludge going to the digesters, a backmixing system is being implemented.

Keywords

Automatic desludging, backmixing, controlled sludge stream, polymer dosing, torque control, turbidity control.

Introduction

The 2020 sludge deregulation is a good opportunity for water companies to re-think their sludge strategy. Being able to free headroom to accept more imports at a reasonable cost will be a key strength in the competition market and it will increase the biogas generation.

Simultaneously to this deregulation, the water companies will have to consider population growth, (and this is particularly true for urban areas) which will bring more wastewater, and consequently generate more sludge that will require treatment. An optimisation of the current assets can make them able to treat larger volume of sludges, and can also extend their life time.

To be ready for the future, wastewater treatment plants (WWTP) will need to increase their automation. With regards to the sludge stream, having a reliable control system able to adapt to changes in the dry solids (DS) of the sludge will reduce the polymer consumption. It will also ensure a stable sludge going to the digester, which will maximize the biogas generation. With a good control of the sludge %DS, blockages in sludge line will also be avoided. The increase automation of the plant will give more time for the operators to focus on others aspects of sludge management, and will also decrease the out of hours calls.

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At Swindon sewage treatment work (STW) a new real time control (RTC) system of the sludge stream is currently being implemented and commissioned. This paper explains the main drivers of this project and describes the different systems present in the sludge stream.

Trial location: Swindon STW

The aim was to find a site in which there was potential for improvement in term of stability of the sludge going to the digester, and biogas generation. To select the location of the first full scale trial of the controlled sludge stream (CSS), different parameters were studied. At the beginning all the sites with belt and drum thickeners were pre-selected. Sites using centrifuge as primary thickening were excluded, as the turbidity of the filtrate following centrifuges is difficult to measure accurately due to the presence of bubbles, and this could make the controlled polymer dosing described below in “Module 3” very challenging. Then the geographical location (proximity with Thames Water borders) and the impact of an increase in the import capacity on the transport costs were considered. Population growth forecasts were also taken into account in the choice of the trial location. Historical data, in particular the stability of the sludge fed to the thickening units and to the digesters were also considered. Following these criteria Swindon STW was selected for the trial.

Swindon STW is located in the Thames Water area and serves 230,000 PE (forecast 2016). 20.5tDS of sludge are digested every day at this acid phase digestion (APD) site. After the screens the effluent is going to 11 Primary Settlement tanks (PST). From there the settled primary sludge (PS) is transferred to a storage tank, before going to two Alpha Laval drum thickeners. Every day, on average 85% of the sludge digested is indigenous, and the 15% are imports coming from various Thames Water sites. Cockpit data are showing variability in the feed to the thickeners and to the digesters.

Controlled sludge stream description

Overview

CSS stands for Controlled Sludge Stream. It is a cluster of technologies aiming at making the plant more reliable, resistant and autonomous. This is achieved via the adaptation of the speed of different pumps based on the sludge characteristics, and the dosing of the polymer based on operating conditions. The control process is not static, but is reactive. This solution allows the operators to control the whole sludge stream, from the PSTs to the digesters. The possibility of choosing the %DS going to the digester, and the savings made on polymer are some of the advantages. Savings are attributable to a better control of the polymer dosing rate, and to the backmixing which allows a proportion of the sludge stream to bypass the thickeners (Masse, 2014). At Swindon STW, it was decided to implement most of the changes on the primary sludge stream. This is because the surplus activated sludge (SAS) stream is much more stable over time than the PS stream (both in term of polymer requirement and sludge thickness). This would have made the CSS implementation on the SAS stream a non-cost effective solution.

A schematic of the controlled sludge stream process is presented on Figure 1 below. In yellow the desludging of the PST, in blue the stable feed to the drum thickener, in orange the polymer dose based on the turbidity of the filtrate and in green the backmixing. The principle of the different modules will be explained further down.




Figure 1: Schematic of the controlled sludge stream at Swindon STW




Module 1: PST desludge pump based on the %DS of the sludge

Description:

PST is a common primary treatment process present in over 90% of Thames Water STW. Located after the screens, it is allowing PS to settle to the bottom of the tank, and to be collected by a rotating scrapper. A desludging pump is intermittently turned on to send the sludge from the blanket to the next step of the treatment. In most of the cases the desludging of the PST starts and ends based on a timer, which can lead to non-optimum operating conditions. Indeed, if the desludging is too long, very thin sludge can be sent to the rest of the process, leading to high volumes of sludge being transferred to the thickeners. Desludging can also be insufficient, in which case a rise of the sludge blanket level can be observed. In some cases, this will increase the mixed liquors suspended solids (MLSS). The augmentation of solids going to the aeration tanks will increase the electricity requirement as well as the quantity of SAS produced. The extra SAS will generate extra costs as more polymer will be required to thicken it, and it might also impact the digestion performances as the SAS is less easily digestible than the PS and more prompt to lead to foaming.As the load to the STW and to the PST is impacted by various factors such as the time (day/night) the seasonality and the rainfalls, setting the optimum timer for each PST to allow for only thick enough sludge to be send to the thickeners has been proven to be challenging.Graph 1 shows the evolution of the DS concentration in the desludge line of one PST during its desludging cycle after a 3h settlement period at Wokingham STW. Over the 4 first minutes the %DS is rather low as there is sludge from the previous desludging cycle remaining in the pipe. Once this sludge has been pumped through, the %DS is higher. After 5 minutes the %DS decrease as the blanket level decreases.

0 2 4 6 8 10 12 14 160.00.51.01.52.02.53.03.54.0

(Wokingham) Dry Solids Content Variation Over the Course of a Desludge Cycle with the Desludge Pumps Operating Under Timer

Control with 3 Hour Settlement Period

Time (min)

Dry

Solid

s (%

)

Graph 1: %DS content variation over the course of a desludging cycle in one PST. Wokingham STW.




A desludging based on the %DS of the sludge being pumped is a simple way to avoid the issues mentioned earlier. It also has the advantage of increasing the life of the assets, as the pumps will be turned on at the same frequency than before the control. Nevertheless, they will run for shorter amount of time. The implementation of this system requires the installation of DS meters on the sludge stream, and a few changes in the software, to allow the desludging pumps to start based on a timer and to stop when the %DS of the sludge falls below a target %DS. For this system to be efficient, the calibration of the DS meters needs to be accurate, for that reason it is advised to have a maintenance plan for the DS meters while implementing this technique. To prevent any issue following a sensor failure some safety parameters should also be introduced on the software controlling the desludging.

Minimum running time set point: to ensure the sludge from the previous cycle and remaining in the pipework is pumped away before the start of the control based on %DS.

Maximum running time set point: to ensure the pump will stop if there is a problem with the DS meter.

Alert if solid meter is showing a constant value during the desludging cycle. This should trig an action for operators to check cleanness and calibration of meter.

Alert if the pumps operate until the maximum running time set point: This should trig an action for the operators to investigate the root cause. These parameters have to be optimised during the commissioning phase.

Others applications

Controlled desludging based on the %DS of the sludge can also be implemented on small sites where the volume of sludge produced has a direct impact on the transport costs (Sindall, 2015). Indeed an optimisation of the PST desludging will reduce the volume of sludge generated, which will lead to a decrease in the tankering frequency. Extended storage of the sludge is lowering the biogas generation capacity; nonetheless the loss in biogas generation due to the sludge ageing is negligible when compared to the savings in transport costs. This is mainly due to the fact that sludge is exported from a multitude of small sites but only represent a low tDS compared to the overall tDS produced by Thames Water.

Module 2: Stable tDS going to the thickeners

Drum thickeners are designed to receive a stable tDS, but usually they are set to work at constant liquid flow, and variations in the feed %DS leads to tDS instability. This situation is not ideal as in that case the polymer dose is not always changing with the tDSin, and the asset is not working at its optimum operating conditions. It might be under or over loaded. The control proposed is very straightforward: Based on measurements taken by a DS meter and a flow meter, the speed of the sludge feed pump will change to deliver a constant tDS to the thickeners.

Module 3: Polymer dosing based on the turbidity of the filtrate

Different method to dose polymer in the thickening units

The major different approaches to the polymer dosing in the thickening units are displayed on Figure2.




In the oldest sites, there is no automatic control and the operators are adjusting the polymer dose manually through the day (based on jar tests) as per Figure 2 A. This technique is not allowing for a constant optimum polymer dose, because the process cannot be constantly monitored. Polymer under dosing generates liquors with a high solid content, which increase the load to the aerated lanes, and increase the energy requirement of the secondary treatment. On the other hand, polymer overdosing engenders extra costs and increases the cleaning requirement as the drums might get blinded by the polymer. In both cases (polymer under or over dosing) the thickness of the sludge is not optimum.

Polymer can also be dosed based on the tDS of sludge going to the thickening equipment. Although this is better than no control at all, it is not taking into account the changes in poly requirement over time (Figure 2 B.)On some more recent sites, the polymer dose is altered based on the %DS of the thickened sludge (Figure 2 C). This is understandably a better solution than the first one, but this method is not taking into account the fact that each sludge, depending on its properties and freshness has a different realistic achievable %DS. Because of the elevated retention time of the sludge in the drum a change in polymer dose can take some time to produce an effect on the thickened sludge %DS. Lastly, it has to be considered that the sludge coming to the thickeners is not of constant quality, and a change in thickened sludge %DS can be attributed to an adjustment in the polymer dose as well as to a variation in the sludge characteristics.

Figure 2: (A) Manual control of polymer dosing; (B) Polymer dosing based on the %DS of the unthickened sludge; (C) Polymer dosing based on the %DS of the thickened sludge;

(D)Polymer dosing based on the filtrate turbidity


(B)

(C)

(A)

(D)



To ensure a process free from the downsides pointed out above it was decided to do the polymer control based on the turbidity of the filtrate. (Figure 2 D) This should increase the reactivity of the system following a polymer dosing change.

Turbidity measurement:

On site, turbidity is measured with the same instrument used to determine the solid content of the sludges, and the accuracy of the measurement can be impacted by different parameters.The first one is the installation of the instrument in the pipework. Indeed if the sensor is inserted in a pipe and is not constantly immersed, or if it is located at the bottom of the line, where it might be blanked with sludge it will not give a reliable result. Another potential challenge for the turbidity measurement is the presence of bubbles in the filtrate. Drum thickeners filtrate is expected to have fewer bubbles than centrifuge filtrate, but they still need to be taken into consideration. As the importance of this phoneme depends on the pipework configuration and on the turbidity meter location, it is complex to predict accurately the impact of the bubbles prior the turbidity meter implementation. While designing the pipework, it was decided to allow for the section with the turbidity meters to be removable to enable the future retrofitting of a baffle box to reduce the bubbling if it was to be too important to take a correct reading.

Finally, to provide a reliable result turbidity meters should be regularly cleaned and calibrated. For ease of operation it is advised to allow for an easy access to the instruments while planning the pipework layout.

Control philosophy:

This is one of the most challenging modifications made at Swindon STW. It is based on the fact that the turbidity of the filtrate varies with the polymer dose. To illustrate the variation in turbidity linked to the polymer dose, lab test were carried out. Raw PS was sampled pre-thickeners (at Reading STW) and different volumes of polymer were added. The turbidity of the filtrate was monitored straight after dewatering (in blue on Graph 2) and samples were send to another laboratory, where the analysis was done more than 24h after the experiment (results in red on the Graph 2). Suspended solids were also monitored, and they show similar trend than the turbidity.

At low polymer concentration the turbidity of the filtrate is very high. This is due to the presence of solids in the filtrate. Up to a certain polymer dosing, further increase in polymer volume leads to an important decrease in the turbidity. Once the optimum polymer dose reached, polymer addition only has a low impact on the turbidity. The instruments have an influence on the absolute value of the turbidity, however here it can be observed that the trend is similar for the two instruments used in the trial.




4 6 8 10 12 14 16 180

5001000150020002500300035004000

050100150200250300350400

f(x) = 861605.045433133 x -̂3.50839233978541R² = 0.939717415313944

turbidity (FTU) Power (turbidity (FTU))Suspended solids (mg/L) turbidity of the filtrate (NTU)

mL of polymer added to 200mL of sludge

Turb

idity

(FTU

) and

susp

ende

d so

lids

(mg/

L)

Turb

idity

(NTU

)

Graph 2: Turbidity of the filtrate versus polymer volume

Graph 2 showed the link between polymer volume in the unthickened sludge and filtrate turbidity. This curve is specific to each type of sludge, and cannot be the only way to control the polymer dose. To allow for a flexible control, the turbidity target should be adjustable by the operator. The control philosophy used in this system is the following:

A- The drum run for a set period of time, the polymer feed pump is set at a constant speed (this start up value can be modified by the operators). As the tDS feed to the thickeners is constant over time, a constant polymer flow is delivering a constant polymer dose (in kgpoly/tDS). This start up period is required to allow the system to reach equilibrium.

B- Based on the comparison between the target turbidity value entered by the operator and the turbidity value read by the sensor, the polymer dosing rate is changed until the turbidity value reaches the target. The software module will aim to provide a sinusoidal dosing pattern around the lowest achievable turbidity.

The theory behind this dosing system is fairly simple, but the full scale application can be a bit more challenging than it initially appears. Indeed at full scale the washwater used to clean the drum thickeners might have an impact on the turbidity. The commissioning period should allow the operator to set the turbidity target while taking this into account. Moreover, it is advised to either insert a baffle box, or leave space for future retrofitting, to cover the case where there will be too many bubbles in the filtrate to enable an accurate reading. This baffle box would have to maintain a sufficient flow to prevent filtrate sedimentation.

This automated dosing technique is tailoring the polymer use for each type of sludge, decreasing the overall polymer consumption. By ensuring an optimum solid quantity in the filtrate, it will enable a better control of the solid load going to the head of the work, and decrease the aeration requirement, leading to energy savings on site.

Backmixing

Principle

All the techniques mentioned above will lead to an increase of the DS of the sludge coming out of the drum thickeners. A sludge too dry is likely to create blockages. Moreover not all the sludge will be thickened to the same %DS (depending on its freshness). To promote digester stability, the digester feed should be stable. Backmixing, represented in green on Figure 1, will enable to decrease the




thickness of the sludge as well as ensuring that the sludge going to the digester has a stable %DS that can be chosen by the operator. The principle of the backmixing is the following: Unthickened sludge is mixed with thickened sludge in the pump at the outlet of the drum thickener. Injection of the unthickened sludge in the pump is allowing for a good mixing. The dilution rate can be calculated either based on torque generated by the thickened sludge pump or on the %DS of the thickened sludge going to the storage tank. Indeed, sampling has shown that there is a good correlation between the torque of the pump and the %DS of the thickened sludge going through it (Masse, 2014).

Control philosophy

Figure 3: Backmixing control philosophy

A schematic of the control philosophy is presented on Figure 3. Prior to the system automation, extensive sampling needs to be done to establish the correlation between the torque of the thickened sludge pump and the %DS of the thickened sludge. The coefficients linking these two parameters are then entered on the software. After that, the operator is able to set up the target %DS and to choose whether he want to drive the backmixing based on torque or based on the %DS value read by the DS meter. The system then calculates the difference between the process value and the setpoint, and adapts the speed of the backmixing pump to decrease the difference between these two values.

As the height of the sludge in the hopper above the pump might impact the correlation between the torque and the real %DS, an ultrasonic level transmitter is located in the hopper to take the sludge height into consideration in the software. This additional parameter will require extra sampling and extra calibration curves.

Operational challenges

When backmixing is driven by the torque of the thickened sludge pump, it relies on constant performance of the pump. A grit accumulation or the build-up of fat coating in the pipework will lead to a change in the correlation between the torque and the %DS and in this circumstance the calibration curve will need redoing. Pump deterioration will also play a role in calibration curve as stator gets worn. A drift of the calibration curve should be taken as an early indicator of a pump or pipework blockage, and as a signal to trigger pump cleaning. Moreover, the energy consumption will be lower if the pipework and pumps are clean.




Overall drum thickener control

In the case of Swindon, it was chosen to give the operator the choice between different options to control the drum thickeners:

Polymer dosing based on the %DS of the sludge coming out of the drum thickener. If that option is selected, the backmixing can’t be turned on as the DS meter is located in the pipework after the unthickend sludge injection point.

Polymer dosing based on the turbidity of the filtrate and backmixing Conventional polymer dosing (only based on the tDS in) and backmixing

Financial benefits for Swindon

In the case of Swindon, it is forecasted that the Controlled Sludge Stream implementation will free enough headspace to allow for the import of 0.94 extra tDS of sludge per day. To study the financial impact of the relocation of these imports, the following parameters should be taken into account: Transport, treatment, biogas generation and disposal of the additional sludge cake.

Cirencester STW is located at 26km from Swindon STW. Every year around 482.5tDS of sludge are exported from Cirencester, mainly to Swindon, Witney, Newbury, Reading, Oxford and Chertsey (by order of importance). As displayed on Figure 4 Swindon is the closest STW, and exporting sludge to the others STW engenders extra costs. Considering a transport cost of 0.24£/m3/km and 0.94tDS of sludge that could be redirected to Swindon each day, the potential savings are £98,409 per year.

Figure 4: Sludge exports from Cirencester to Swindon (26km), Newbury (69km) and Chertsey (127km)

In some of these others sites, such as Newbury, the sludge is limed instead of being digested. Liming cost are higher than digestion costs, and redirecting sludge exports from Newbury to Swindon could save up to £13,400/year.

The additional gas generation is estimated at 315m3/day. Taking into accounts the electricity generated and the ROCs, the power generation is estimated at £20,700/year.




These additional imports would also generate additional sludge cake that will need to be disposed of. The cost associated with the disposal of the extra-sludge cake was estimated at £17,200/year.

Overall, the total savings are estimated to be £115,000/year.

Conclusion

The main benefit of the control sludge stream implementation at Swindon is due to its particular location at the border of the Thames Water area. Currently a lot of sludge is being tankered to Newbury, where it is limed. Bringing this sludge to Swindon would reduce the treatment and transport cost and increase the biogas generation. Control sludge stream implementation is also expected to generate others savings, indeed the optimisation of the polymer dosing should decrease the polymer requirement. The backmixing will also allow a part of the stream to bypass the thickener and thus decrease the polymer consumption. Moreover, the control of the filtrate turbidity will ensure that the liquors returning to the head of the plan are not too concentrated. Lastly, the CSS will ensure a stable and reliable sludge stream. This will free operators’ time, and the stable feed to the digester will increase the biogas generation.

Automated systems can bring more autonomous, resilient and stable systems, but they will only perform where the instruments are calibrated and are maintained regularly. Maintenance and checks are crucial, and should be planned as part of the project

The automatic desludging of the PSTs based on the %DS of the sludge is now installed at Swindon STW and it is currently being commissioned. All the work around the drum thickeners is due to start shortly.

In the case of Swindon STW all the technologies described in this paper are implemented, but the controlled sludge stream should be adapted to each plant to ensure the most cost effective solution is implemented. Controlled system must be tailored with regards to site specifics. THP sites can for instance implement backmixing pre-THP, to reach the 16%DS required by the process.

References

Masse, A-L., Perrault, A., Fountain, P., Thomas, M., Thames Water Utilities, UK, 19th European Biosolids & Organic Resources Conference & Exhibition, Nov 2014, Manchester

Sindall, R. and Nikolova-Kusco, R., Thames Water Utilities, UK 9th European waste water management conference & exhibition, 12-13 Oct 2015, Manchester



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