Retrofitting of a Full-Scale Dewatering Operation for Industrial Polymer Effluent Sludge

Abstract

This paper presents a comprehensive study on the redesign of a dewatering process in a polymer sludge wastewater treatment plant. The study focuses on addressing the challenges posed by high levels of organic pollutants in the sludge, with the aim of enhancing dewaterability, reducing odors, and minimizing suspended solids. Initially, a vacuum belt filter was utilized, but it proved inadequate in removing sufficient water, resulting in substantial annual disposal costs. To address this issue, a filter press system was proposed, which significantly improved the dewatering process, producing a dryer cake with a solid content of 35%, compared to 19% achieved by the vacuum belt filter. Performance evaluation of the processes was conducted based on concentration of solids and capture efficiency, demonstrating the superiority of the filter press method. Furthermore, the filtrate obtained through the filter press met local discharge regulations, eliminating the need for additional treatment before disposal. The implementation of the filter press not only improved the dewatering process and ensured compliance with discharge standards but also resulted in substantial cost savings of up to 50% per year. The payback period for the current system was determined to be 1.5 years, highlighting the economic advantages of the filter press. Overall, the findings of this study emphasize the practical advantages of the filter press in handling sludge cakes and filtrate for disposal, making it a favorable choice for dewatering polymer sludge and other wastewater treatment plants.

1. Introduction

The treatment of industrial wastewater is highly crucial as it is meant to decrease the existence of pollutants in the water that is discharged, thereby ensuring the preservation and quality of water resources as a whole [1]. With the emergence of new industries and products, like pharmaceuticals and personal care products (PPCPs), disinfection by-products (DBPs), and per- and poly-fluoroalkyl substances (PFAs), the range and complexity of pollutants present in wastewater have grown considerably, posing greater challenges for treatment processes [1].

Several wastewater treatment processes, including primary treatment, secondary treatment, biological nutrient removal (BNR), waste stabilisation ponds (Lagoons), and anaerobic and aerobic digestion, generate substantial amounts of sludge, typically consisting of over 90% water. Multiple methods can be employed for water and wastewater treatment, including chemical coagulation–flocculation, electro-coagulation, electro-floatation, sedimentation precipitation, extraction, evaporation, membrane filtration, activated carbon adsorption, ion-exchange, oxidation and advanced oxidation, incineration, biodegradation, and electrochemical treatment [2].

Dewatering sludge involves the separation of water from solid sludge material to reduce its volume, making it easier to handle and dispose of. The selection of a dewatering method depends on various factors, including the desired level of dewatering, sludge characteristics, availability of space, budget constraints, as well as considerations for regulatory compliance and environmental impact. For instance, centrifugation utilises centrifugal force to remove water, while filter presses apply pressure to facilitate water flow through filter cloths. Consulting with experts and conducting feasibility studies is crucial in determining the most suitable dewatering approach for a specific sludge application. The goal is to remove free water in the sludge and achieve a significant reduction in volume. Bound water, on the other hand, is more tightly bound to the sludge particles and is not easily removed through conventional dewatering methods [1]. In terms of dewatering, mechanical processes are generally preferred for thermal drying, drying beds, filter press, screw press dewatering, geotextile dewatering and belt press filtration, due to economic considerations [2]. There are various dewatering methods and devices extensively utilised in the industry, including centrifugation, filter presses, belt filter presses, rotary drum filters, decanter centrifuges, screw processes, vacuum filtration, drying beds, thermal drying, and vibration and mesh plates. In the case of vibration and mesh plates, the mixture is subjected to vibration, aiding in the separation of solids from the liquid, while the mesh plates function as filters, enabling the passage of the liquid while retaining the solids [3]. Among these, centrifuges and belt filter presses are the preferred tools for large-scale sludge dewatering projects, while electro-osmosis-assisted sludge dewatering technology stands out as one of the most extensively researched and promising approaches. In general, electro-osmosis is paired with pressure filtration methods, like belt filtration, plate-and-frame filtration, and diaphragm filtration. The crucial step in sludge treatment prior to disposal is reducing the sludge volume through water separation, aiming to minimise transportation and handling costs.

The dewatering level depends on the type of industry and specific technology employed. While conventional sludge dewatering processes typically achieve a final dryness of around 20% dry solids (DS), fibrous sludges, such as pulp and paper sludge, can reach higher levels of 50–60% DS. Key design parameters for belt filter presses include requirements for chemical conditioning, hydraulic and solids loading rates, and belt cleaning. Sludges originating from industries, such as mining, pulp, paper, and textiles, have the potential to produce dewatered cakes with significantly high solids concentrations. To evaluate the advantages and disadvantages of three dewatering techniques, Table 1 provides a comparison between the filter press, and vacuum belt filter.

In the operation reported in this paper, sludge is produced as a result of breaking down the colloidal emulsion; this substance is referred to as agglomerated destabilised colloidal particles. A wide range of materials have been used as physical conditioners, including char [4,5,6], coal fines [7], biomass [8,9], fly ash [10], cement kiln dust [11], gypsum [5,12], and quick and hydrated lime [13]. The benefit of using a skeleton builder is often evaluated using volume of the filtrate, cake solids content, filter cake properties, such as cake porosity, permeability, cake yield stress [14], and some other parameters, such as the specific resistance to filtration (SRF) [4,15], and the net sludge yield [16].

In general, the efficiency of sludge dewatering is affected by the sludge type, conditioning of sludge, dewatering device, and operating conditions. It is commonplace to find filter cakes with a moisture content of around 80% [17]. Dewatering and disposal of waste sludge are major economic factors in the operation of wastewater treatment plants, as the associated costs are included in the production cost. Consequently, these costs can have a significant impact on the sale price and market competition [18]. The literature reports a number of studies on sludge dewatering, but the various methods used report high water contents and cakes that require additional handling. Additionally, no case study has been seen dealing with polymer sludge dewatering.

In light of the foregoing, this paper presents a case study of replacing an industry standard dewatering method for polymer sludge using vacuum suction over a conveyor belt, with a more efficient filter press. The filter press will be shown to produce dryer cakes with a moisture content markedly less than previously reported. A diatomaceous earth layer on the filter press plates ensures the cakes are spontaneously dislodged. The filtrate from the filter press met discharge water parameters and was directly disposed of without further treatment. The associated cost savings are also presented and discussed. Overall, implementing the filter press system make it a viable alternative for sludge dewatering in polymer sludge (and other) wastewater treatment processes.

Table 1. Assessment of the pros and cons of typical dewatering techniques.

Equipment	Advantages	Disadvantages
Filter press	Amounts of dissolved solids in sludge cake can be high High-quality filtrate Can be applied for a wide range of sludge types High capacities can be achieved without a drastic increase in surface area	Operation can only be in batch mode. Investment and costs of operation can be high Skilled personnel and frequent meantenance required Conditioning with inorganic chemicals required and can produce more solid materials.
Vacuum belt filter	Lower capital, operating and energy costs Easier to maintain. Acceptable filtrate quality	Can be difficult to shut down due to conditioned sticky sludge [18] Sensitive to the characteristics of input sludge, type of polymer and the dosage. Large amounts of belt wash water are required Can result in corrosive products Replacement of torn belts is difficult Losses in belt tension and wash water pressure by ageing

Table 1. Assessment of the pros and cons of typical dewatering techniques.

Equipment	Advantages	Disadvantages
Filter press	Amounts of dissolved solids in sludge cake can be high High-quality filtrate Can be applied for a wide range of sludge types High capacities can be achieved without a drastic increase in surface area	Operation can only be in batch mode. Investment and costs of operation can be high Skilled personnel and frequent meantenance required Conditioning with inorganic chemicals required and can produce more solid materials.
Vacuum belt filter	Lower capital, operating and energy costs Easier to maintain. Acceptable filtrate quality	Can be difficult to shut down due to conditioned sticky sludge [18] Sensitive to the characteristics of input sludge, type of polymer and the dosage. Large amounts of belt wash water are required Can result in corrosive products Replacement of torn belts is difficult Losses in belt tension and wash water pressure by ageing

2. Materials and Methods

2.1. Sludge Characteristics

The sludge utilised in this study was obtained from Scott Bader’s effluent treatment plant (ETP), where it underwent a chemical coagulation and flocculation process and settled in a conical sedimentation tank. The sludge analysed in this study consisted of a mixture of water and solids. On a dry basis, the sludge contained approximately 9% solids, including polymer, Aluminium Hydroxide, Calcium Sulphate, and traces of other substances. The fundamental characteristics of the sludge were determined according to standard methods [19,20,21,22]. The sample exhibited a high chemical oxygen demand (COD) level exceeding 40,000 mg·L⁻¹, indicating the presence of organic pollutants. The concentration of total suspended solids (TSS) in the same sample, assessed using the dried residue weight on the filter paper, remained as 10,500 mg·L⁻¹. The solid contents of sludge were measured through a Halogen lamp based digital moisture analyser as 9% on a dry basis. Using an electrometer, the sample’s pH was determined to be 8.5. The visual appearance of the sludge was that of a thick and white liquid, and the density hydrometer showed a density of around 1.2 kg·m⁻³. More detail on the characteristics of the sludge have been reported in an earlier paper co-authored by the lead author [2].

2.2. System and Procedure

2.2.1. Vacuum Belt Filter

The flocculated effluent is continuously passed through a gravity clarifier for sedimentation in which clear water rising up is discharged from the top, whereas settling flocs creating a sludge blanket at the bottom are periodically desludged to prevent the flocs from being carried along with discharge water. The sludge is stored in a conical hopper until sufficient volume is available for processing through a vacuum filter. The sludge is then transferred into a mixing tank, where lime and steam are added before pouring it on the belt. Steam is added in the sludge for thermal conditioning to release bound water from solids, thereby enhancing its dewaterabilty [7]. Additionally, lime effectively reduces odours and suspended solids in the filtrate, by converting sulphides in solution, transforming hydrogen sulphide into non-volatile sulphide and bisulphite ions at alkaline pH levels. Lime for sludge neutralisation and stabilisation reduces pathogens, improves sludge handling properties, and enables long-term storage without odour formation. This is crucial because untreated sludge can degrade within a few days, producing highly toxic hydrogen sulfide gas. Therefore, it is important to reduce the storage duration and dewater the sludge before it starts producing toxic gas. Furthermore, the inclusion of lime enhances sludge porosity, which facilitates water removal processes [8].

After conditioning, the sludge is processed through a vacuum belt filter using a moving horizontal belt of filter medium. A suspension of solids is fed on to the upper surface of the belt close to one roller. The drive roller moves the belt forward to drop the sludge cake into the skip as soon as it moves backward to its original position. At this resting position, the vacuum pump starts generating a vacuum again for the next suction cycle, and the vacuum is released during the forward movement to release the sludge cake from the belt. During this forward and backward movement of rollers, the contaminated belt is jet-washed for unblocking the pores. The cake formed in the feed zone is carried from dewatering to drying and discharge zones as the belt moves forward. Figure 1 shows the block flow diagram of the sludge dewatering system.

2.2.2. Performance Variables for Vacuum Belt Filter

The performance of the vacuum belt in the dewatering processes is evaluated based on various factors. These include the percentage of solids present in the sludge cake, the efficiency of solids captured during the filtration process, the rates of solids and hydraulic loading, and the appropriate dosage of polymer required for effective dewatering. The performance is affected by several variables, such as belt speed, belt tension, belt porosity, vacuum set point, steam injection rate and lime flow rate. These variables play a crucial role in determining the overall effectiveness and efficiency of the vacuum belt dewatering process for optimal results in separating water from the sludge and achieving the desired level of solids concentration in the cake.

The vacuum belt is outfitted with fixed-speed vacuum pumps that maintain a steady −0.3 bar vacuum pressure beneath it for effective sludge filtration. The vacuum belt filter utilises both vacuum and gravity forces for quick separation of liquid and solids. A high belt speed reduces the retention time of sludge on the belt, which provides insufficient time for suction and dewatering. Slow belt speed produces better results but requires longer time to treat the sludge volume. The belt speed is adjusted to provide reasonably dry sludge cake with the ability to treat the sludge within a couple of days of storage in order to avoid the production of toxic gases. Figure 2 is a schematic of the vacuum belt filter.

2.2.3. Material Balance for Vacuum Belt Filter

Total sludge processed through the vacuum belt filter is 108 m³ per month for a run time of 50 h, which is equivalent to a dewatering rate of 2160 litres per hour. The lime solution and steam added to the sludge contribute 12% of the total mixture poured onto the vacuum belt. The concentration of the lime solution used for conditioning is 20%. The material balance calculation was carried out as follows:

$$F_{sl}=F_f+F_c+F_{st}+F_{Li}$$

(1)

$$x_{w,sl}{ F}_{sl}={x_{w,f} F}_f+x_{w,c} F_c+x_{w,st}{ F}_{st}+{x_{w,Li} F}_{Li}$$

(2)

$$y_{s,sl}{ F}_{sl}={y_{s,f} F}_f+y_{s,c} F_c+y_{s,st}{ F}_{st}+{y_{s,Li} F}_{Li}$$

(3)

Equation (1) is the total material balance, while Equations (2) and (3) are the component balances for water and solids, respectively, where F is the volumetric flow rate of the component identified by the subscript (sl for sludge, f for filtrate, c for cake, ww for washing water, and Li for Lime), and $x_w$ and $y_s$ are the volume fraction of water and solids, respectively. However, the main unknown of interest is $y_{s,f}$, the fraction of solids in the filtrate. While samples are taken regularly and tested for total solid content, the material balance equations allow values to be determined depending on the feeds to keep a tight control over the filtrate quality. Laboratory characterisation usually confirms that the calculation and experimental results usually have a difference between them of around ±5%, with experimental uncertainties considered.

Figure 3 shows the mass balance of sludge and cake around the vacuum belt filter. The lime becomes part of the sludge cake, whereas the water, filtrate, and condensate collected in the vacuum system are contaminated and cannot be sent to the sewer. This filtrate and contaminated jet washing water create a recycle stream that is sent back to the effluent treatment plant for chemical treatment.

The amount of jet wash water is quite significant but equally important in unblocking the pores of the belt, maintaining its efficiency during rotation and constant use as a filter medium. The solid contents of the sludge and sludge cake are 9% and 19%, respectively.

2.2.4. Filter Press

A filter press is a type of mechanical separation equipment used in various industries to separate solids from liquids. It operates on the principle of pressure filtration, where a slurry (a mixture of solid particles and liquid) is pumped into the filter press and subjected to pressure, causing the liquid to pass through a filtering media, forming a cake, which can then be removed. The plate-and-frame press is a batch device that has been used to process difficult-to-dewater sludge. Recent improvements in the degree of automation filter media and unit capacities have led to a renewed interest in pressure filtration for application with industrial sludge. The ability to produce a very dry cake and clear filtrate are major points in favour of pressure filtration, but they have a higher capital cost than vacuum filters. Their preference over vacuum filters is based on economic considerations. The full filter press dewatering operation and the material balance around the filter press are depicted in Figure 4 and Figure 5.

The filter press used as a trial in the study had a capacity of 342 L in its 25 chambers with 26 plates in total, each 800 by 800 mm in dimension. The maximum air pressure available at the proposed area was 6 bar. Therefore, a pressure booster, which is a mechanical system designed to raise the pressure of compressed air, was employed to increase this from 6 to 8 bar. A pressure booster device functions by taking in a fluid, or often air or gas, at a lower pressure through an inlet. This pressurised fluid is then discharged at a higher pressure through an outlet. Some boosters include controls to regulate the output pressure. The increased pressure allows the fluid to perform specific tasks, such as powering tools or driving actuators in industrial applications. The filtration cycle was preceded by pre-coating the filter press with 500 L of 2.0% diatomaceous earth solution. Diatomaceous earth (DE) powder, derived from fossilised diatoms, which are microscopic algae with silica skeletons, offers numerous features and benefits. Its high porosity and abrasive nature make it ideal for absorbing liquids, filtering fine particles, and acting as a natural insecticide. DE is widely used as a desiccant. The slurry was pumped through the filter cloth for few minutes, during which the particles are trapped on the cloth and make a coating to keep it from blinding off, due to the small size of the particles being filtered. The filter cloth is coated with a porous coated medium and the pore spaces of diatomaceous earth are used as filter [9]. The trial filter press used for the present study is shown in Figure 5.

2.2.5. Material Balance for Filter Press

The quantity of sludge dewatered in a 3-h cycle to produce 342 L of sludge cake and 991 L of filtrate is 1333 L. Figure 6 shows the material balance around the filter press based on dewatering monthly sludge volume of 108 m³. The material balance calculation was carried out as follows:

$$F_{sl}=F_f+F_c$$

(4)

$$x_{w,sl}{ F}_{sl}={x_{w,f} F}_f+x_{w,c} F_c$$

(5)

$$y_{s,sl}{ F}_{sl}={y_{s,f} F}_f+y_{s,c} F_c$$

(6)

The main difference between between Equations (4)–(6) and (1)–(3) are the absence of terms for lime and washing water as shown in Figure 7, which simplifies the process when compared to Figure 4. Comparing the −0.3 bar vacuum pressure under the vacuum belt filter, 8 bar pressure was applied in the filter press to dewater the sludge. As a result, the filter press produced dryer cake with solid contents of 34%, compared to 19% from the vacuum belt filter. The filtrate produced from the filter press was clear of any visible contamination, and the total solids showed that it met all the discharge water parameters agreed upon with the local authority (50 mg/L, far below the UK threshold of 150 mg/L). Therefore, the filtrate was sent straight to the sewer without the need for any further chemical treatment.

3. Results and Discussion

3.1. Analysis of Vacuum Belt Operation

The total solids content of the sludge cake is influenced by two key factors in the sludge treatment process: the length of time the sludge remains on the belt (determined by the feed rate and belt speed) and the negative pressure applied beneath the belt (vacuum). A longer residence time and higher negative pressure result in a drier sludge cake, while a shorter residence time and lower negative pressure lead to a moister sludge cake. The residence time of the sludge on the belt was adjusted by varying the feed rate and belt speed, with the vacuum pressure maintained at a constant −0.3 bar during data collection.

Decreasing the feed rate and belt speed resulted in an increase in the percentage of total solids within the cake. Figure 7 illustrates the removal of total solids using five different belt feed rates on the vacuum belt. Error bars were obtained as an average deviation of a triplicate of measurements, which shows that they are within 2% of each other. The patterns of belt speed rates are clearly comparable. As expected, a higher sludge feed rate resulted in a poorer sludge cake for all belt speed conditions due to a lower residence time. Conversely, a lower sludge feed rate produced a drier sludge cake.

To meet the dewatering requirements within the allocated operating hours, the vacuum filter belt was operated at a speed of 30 m per hour with a sludge feed rate of 2160 litres per hour. Under these operating conditions, a sludge cake with a solid content of 19% was achieved. Furthermore, the analysis shows that a solid content of up to 23.7% could be achieved in the sludge cake by reducing the feed rate to 1200 L per hour, but this would require a 44% increase in the run time, which is not desirable.

3.2. Analysis of Filter Press Operation

Compressed air pressure for the sludge feed pump and filtration duration were varied during the filter press trials to observe their effect on sludge cake. A 2-inch air operated double diaphragm (AODD) pump was connected to the available compressed air supply coming from air compressors rated for 6 bar. A pressure regulator was used to change the supply pressure between 3 and 6 bar for the sludge feed pump but all these initial tests failed to produce a solid sludge cake, indicating the need of higher pressure. Up to 8 bar pressure was achieved through the pressure booster, which led to successful trials.

As expected, significant variations in the backpressure and filtrate flow rate were observed at the start and end of the filter press cycle. The cycle was started with a clean filter cloth and empty plates, which provided very little pressure drop at the beginning across the whole unit. During the first 10 min, up to 3250 L·h⁻¹ flow rate of filtrate was observed, starting to drop rapidly afterwards, as the solids started to deposit on the surface of the filter cloth, resisting the flow of liquid.

A high pulse rate for the AODD pump was observed during the first 10 min, which also started reducing with the build-up of solids between the plate chambers. As the space between the filter plates filled up with more sludge and the filtrate had a thicker layer to pass through, the feed rate of sludge to the press, as well as the flow rate of filtrate from the press, decreased, whereas the pressure in the filter press increased. After about an hour of filtration cycle, the pressure in the filter press came to a steady point of 7.4 bar with no further remarkable increase; however, the filtrate continued coming out of the press for another hour before the flow rate dropped to below 250 L·h⁻¹.

As shown in Figure 8, after 3 h of filtration cycle, the following observations were made; a trickling flow of filtrate below 50 L·h⁻¹, steady pressure at 7.4 bar and a reduced rate of 2 pulses per minute from the AODD pump. The hydraulic ramp pressure was released, and the solid cake dropped down into the skip as soon as the plates were opened. As shown in Figure 9, the appearance of the sludge cake produced from the filter press was much drier and harder compared to that from the vacuum belt filter. The solid contents of this sludge cake were observed to be 35%, with no remarkable increase when the cycle time was increased beyond 3 h.

3.3. Analysis of Filtrate from Vacuum Belt Filter and Filter Press

The same analytical methods were used for the analysis of filtrate from the vacuum filter belt and filter press, as mentioned in Section 2.1 for sludge characteristics. Comparing the results from the analysis of the vacuum filter belt as a base case, the filter press provided further reductions of 87%, 78%, 57% and 55% in chemical oxygen demand, total suspended solids, total solids and turbidity, respectively. As shown in

Table 2. Comparison of vacuum belt filter and filtrate effluent characteristics.

Characteristics	Units	Sludge	Vacuum Belt Filter Filtrate	Filter Press Filtrate	Reduction
Total Suspended Solids (TSS)	mg·L−1	>10,500	1125	248	78%
Total Solids (on dry basis)	%	9	0.75	0.32	57%
Chemical Oxygen Demand (COD)	mg·L−1	>20,000	1250	165	87%
pH	-	8.5	9.3	8.4	10%
Turbidity	NTU	>1000	200	90	55%
Sludge Cake Water Contents	%	-	81%	65%	20%

below, the filtrate from the filter press meets all discharge water parameters, indicating that it can be sent to the sewer without any further chemical treatment.

Furthermore, the pH of the filtrate, obtained through the removal of water from the sludge, remains consistent with the pH of the sludge itself, as no acidic or basic substances are introduced to modify the pH during this process. The pH remains unaltered due to the presence of DE powder. Furthermore, the pH of the discharge water from the filter press falls well within the agreed upon consent limit of 6–10, as established by the local water authority. It is worth noting that the pH range of 8–9 is maintained during the final stage of wastewater treatment for the purpose of flocculation. Consequently, both the pH of the sludge and the filtrate fall within this specified range.

The dewatered sludge is collected by external waste management contractors. As it is non-hazardous waste, there is no limitation on what it can be used for. To the best of the authors’ knowledge, most use it for landfilling.

3.4. Cost Comparison of Vacuum Belt Filter and Filter Press Operation

Various factors were considered when estimating the running cost of the vacuum belt, taking into account in the estimation of vacuum belt running costs. One of these factors involved measuring the amount of steam consumed in the mixing tank by collecting the condensate. A 20% lime solution was prepared in the effluent treatment plant and recirculated around the vacuum belt filter to prevent pipe blockages. The volume of water jet used to clean the belt was measured to determine the cost of clean water. On the other hand, the cost estimation for sludge dewatering included the chemical treatment cost for contaminated water and the cost of clean water for filtrate, whereas the chemical treatment cost was incurred for contaminated water and filtrate. The power consumed by the mixing tank’s agitator, vacuum belt drive motors, vacuum pump, and recycle pump was also included in this estimate as miscellaneous costs. The sludge cake was transferred to a 5-tonne skip for disposal as non-hazardous waste. The collection, transportation, and disposal costs of the sludge cake were obtained from a waste management company. The total annual cost of dewatering the sludge from the vacuum belt filter was estimated to be 150,382 GBP, which equates to 1392 GBP per m³ of sludge.

The efficiency of the filter press has demonstrated a significant improvement compared to that of the vacuum belt filter. The main advantageous aspect of the filter press over the vacuum belt filter is higher compaction and solid contents, producing 59% less sludge cake for disposal. The quality of filtrate from the filter press allowed direct discharge to the sewer, which saved on treatment costs. The total annual cost of dewatering the sludge from the filter press was estimated to be 76,006 GBP, which equates to 704 GBP per m3 of sludge. This comparison (

Table 3. Comparison of Sludge Treatment Cost using Vacuum Belt Filter and Filter Press.

Parameter	Units	Vacuum Belt Filter	Filter Press
Sludge Throughput	m3/month	108	108
Sludge Solids	% (on dry basis)	9	9
Sludge Cake Solids	% (on dry basis)	19	34
Sludge Cake produced	m3/month	51	28
Treatable Filtrate Produced	m3/month	155	0
Filtrate Treatment Cost	GBP/month	1240	0
Sludge Disposal Cost	GBP/month	9693	5719
Miscellaneous Cost	GBP/month	1257	435
Total Monthly Cost	GBP/month	12,532	6334
Total Annual Cost	GBP/year	150,382	76,006
Sludge Treatment Cost	GBP/m3	1392	704
Filter Press Savings	GBP/year	74,376

) shows potential annual savings of 74,376 GBP by using the Filter Press, which is almost 50% of the treatment cost with the existing system.

3.5. Filter Press Size Selection

Based on the size of the trial filter press and a cycle time of 3 h, 81 cycles of operation are required to process 108 cubic meters of sludge per month. Figure 10 illustrates the selection of filter press size. This is equivalent to 4 cycles daily in order to effectively manage the entire volume of sludge generated during the effluent treatment process. Therefore, a second shift is required every day, which would result in an undesired increase in operating costs. To avoid increased operating costs for future expansion, a 750-L filter press was constructed using the frame of a 1000-L filter press. This allows for expanding operations by simply mounting additional plates.

3.6. Project Equipment and Cost for Change to Filter Press

Figure 11a shows the completed filter press. A 94,000 GBP budget was determined to be sufficient for the project, including installation, commissioning, startup and training expenses. An additional 10,000 GBP was required to construct a new foundation, ensuring its stability in supporting the weight of the filled filter press, which is approximately 7 tonnes. The anticipated payback period for this investment was calculated to be 1 year and 5 months, estimated as the difference between the monthly sludge disposal costs of the two methods (£9613 − £5719 = £3894), which takes around a year and half to achieve the difference in total annual costs of the two methods (i.e., £74346).

The installation included a raised platform for accessing the filter press, a pendant control system to open and clean the plates individually, a local control panel for operation, a laser curtain for safety, a hanging curtain for cleaning, a pre-coat tank for DE solution preparation, and a filtrate tank for collecting discharge water samples (shown in Figure 11b).

4. Conclusions

This paper has presented a case study that shows substantial improvements in the dewatering process in a polymer sludge wastewater treatment plant by replacing a vacuum belt with a filter press system. The vacuum belt method was shown not to remove sufficient water and costs are incurred in water disposal yearly. However, with a filter press, only 66% of the cake is water, hence producing a dryer cake with a solid content of 34%, compared to 19% from the vacuum belt filter. Overall, the performance of the dewatering processes was evaluated based on solids concentration, solids capture efficiency, and dosage of polymer, with the filter press method shown to produce water well below the local legal limit for direct surface water disposal. Additionally, the filter press offers substantial cost savings, estimated at 74,376 GBP per year, when compared to the vacuum belt filter, a 50% annual decrease over the vacuum belt system.

The size of the filter press and the number of cycles necessary for managing the sludge volume play a crucial role in operational efficiency. The total budget for selecting, designing, and installing the filter press, including foundation construction, was 94,000 GBP, and the payback period was estimated to just under 1.5 years. The cost of 94,000 GBP mentioned pertains solely to the filter press and does not encompass associated expenses, such as the gantry, civil work, installation, pipework, tanks, pumps, and other relevant components. In conclusion, implementing the filter press system provides enhanced efficiency, cost savings, and higher-quality sludge cakes compared to the vacuum belt filter, significantly improving the overall sludge treatment process, meeting discharge water standards, and reducing operational expenses.