Reprinted from July 202043HYDROCARBON ENGINEERING

T he rising costs of solids disposal demands greater performance and efficiency from the solids dewatering units in wastewater treatment plants. With 40+% of the operating budget tied up in the

cost of disposing of solids, inexpensive improvements and mechanical optimisations to the gravity drainage section (GDS) can result in significant savings by improving the overall performance of belt filter presses (BFPs) – and this is with no costs after the installation and implementation of the upgrades.

Why the belt filter press?BFPs treat solids containing sludge from processes as varying as mineral solids from mining sites to biological sludge generated from municipal waste treatment and everything in between. While BFPs can come in an almost infinite number of designs and configurations, each press contains the same three sections: GDS, wedge section, and pressure section.

Each section plays a vital role in the removal of water from the treated sludge, but the majority of the time spent troubleshooting and optimising BFPs revolves around the pressure section and dewatering aide (polymer) optimisation. However, the biggest cost advantage is found in optimising the GDS. With greater than 50% of the total water removed during the entire process removal occurring in the GDS, significant improvements in overall performance are generated with minimal capital and potentially no other changes to the system operationally.

In two separate wastewater systems, Athlon audited BFPs in order to improve the produced cake dryness and reduce the site’s overall spend surrounding the BFP systems. One system consisted of several rows of plows used to shift the treated sludge around the GDS and expose more belt

Troy Davis, Athlon, a Halliburton Service, USA, presents an overview of

how a belt filter press can be optimised to reduce the cost of solids disposal.

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for additional free water drainage, but used an inadequate sludge delivery and distribution header that resulted in the solids piling up in the centre of the GDS belt. This poor distribution of the treated sludge resulted in less than ideal free water drain across the GDS and less than ideal dewatering in the pressure section. In the second system, plows were not present to shift the treated sludge and expose belt for enhanced free water drain, resulting in significant amounts of free water requiring removal in the pressure sections. Both of these systems experienced poor dewatering and lower cake solids percentages, meaning tons of water normally removed during the BFP process was hauled to landfills at additional costs to the site.

Optimisation of these GDSs resulted in almost 10% improvement in dewatering efficiency in both systems. Depending on the performance of the BFP prior to the optimisation, this improvement in cake solids can result in a reduction of between 10 – 16% in overall tons shipped off site and a reduction of between 12 – 18% in water shipped off site. In both of these sites, the BFP optimisation resulted in greater than US$50 000/yr of savings in shipping costs with solutions that cost fractions of that.

Importance of dewateringDewatering is defined as the removal of excess water from wastewater treatment plant (WWTP) generated solids in order to reduce their volume and to produce a product for further processing or disposal. Dewatering differs from sludge thickening not in the processes, procedures, and even equipment used in some cases, but rather in the product generated. Sludge thickening results in substantially higher solids concentration, and in turn less water, but the final product still flows and moves like a liquid. Dewatering sludge results in a non-fluid cake that looks and behaves like a solid, namely that it is not free flowing in nature.1,2,3

The purpose of expending time, energy, and money dewatering solids lies in the fact that dewatering dramatically reduces the weight and volume of wastewater produced solids that require offsite disposal or treatment. Water generally composes greater than 99% of the mass accounted for in the sludge wasted from a biological treatment unit (WAS), and even the most effective and efficient clarifiers only generate WAS with moisture water contents of 98.5%.2,3 Sludge disposal can already occupy 40 – 50% of a WWTP’s budget with dewatering occurring, and this percentage significantly increases as dewatering performance decreases.4 To illustrate the importance of effective dewatering, Figure 1 depicts the impact of increasing solids concentrations from 1% up to 30%.

In addition to disposal costs comprising one of the largest expenditures for WWTPs, the costs to dispose of the sludge have increased across the industry. The average cost to dispose of 1 t of solids increased to US$55.36 in the US, a 3.5% increase from 2018 to 2019.5

Belt filter press basicsWhile there are numerous pieces of equipment to dewater sludge generated in a wastewater treatment plant such as centrifuges, filter presses, and drying beds,the remainder of this article will focus on BFPs.

BFPs consist of two tightly woven belts that wrap around permeable rollers of differing radii while under tension in order to press and squeeze the free and most of the interstitial water from the solids floc formed by the proper chemical treatment.1,2,3 To further break BFPs down, each press consists of three different sections that all play significant and unique rolls in the effective dewatering of the generated sludge:

n GDS. n Wedge compression section or moderate pressure section. n High pressure section.

The GDS serves to remove the majority of the free water by allowing the treated sludge to distribute evenly over the belt and by moving/turning the sludge by using plows which shift the sludge side-to-side, freeing up uncovered belt and allowing free water to bypass flowing through the sludge solids. The wedge compression section applies slight pressure to the material after the filter section, further removing the remaining free water, but most importantly begins to fix the sludge in place prior to the pressure section. This helps prevent the sludge from flowing out the sides of the belt under higher pressure. In the high pressure section, the now fixed sludge cake experiences significant pressure, which causes any remaining

Figure 1. Impact of effective dewatering on water removal from sludge.

Table 1. Percentage improvement realised by optimising the distribution of the belt filter pressCake solids

lb solids/t cake

lb H2O/t cake

t cake required to remove 1 t solids

% reduction in t disposed of with 1.3% cake increase

lb H2O shipped to remove 1 t solids

% reduction in lb H2O removed with 1.3% cake increase

15.97% 319.4 1680.6 6.26 10 523

17.30% 346 1654 5.78 7.69 9561 9.15%

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free water and significant amounts of interstitial water to be pressed out of the solid cake.

Significant efforts are paid to the high pressure section when troubleshooting and optimising BFPs; however, the GDS removes the majority of the water in terms of lb and can have the largest impact in terms of performance of the other two sections downstream of it. As seen in Figure 1, an increase in sludge concentration from 1% to 2% results in 50.5% reduction of water remaining in the sludge. Properly operating and optimised GDSs can double, triple, or even more so increase the solids concentration in the sludge prior to the wedge

compression section. It is common to see solids percentages ranging from 5 – 7% after effective treatment in the GDS. If the incoming sludge contained 2% solids, achieving a 5% solids concentration entering into the wedge section means a 61% reduction in lb and volume seen by the wedge compression and high pressure sections. A 7% solids concentration means a 73% reduction. These significant reductions in free water entering the wedge compression and high pressure sections means that the solids are less likely to press out the side of the belt and that the press utilises more of the energy imparted on the sludge to press out interstitial water, as the free water is not present.

Optimising the GDSOptimising the GDS generally consists of two modifications:1. Altering sludge distribution to ensure even placement of sludge onto the GDS.2. Altering the plows to maximise belt available for free water drainage.

By ensuring sludge distribution onto the gravity section occurs in an even, consistent manner, solids build-up will be equal across the entirety of the belt. This even distribution will result in almost uniform thickness of solids across the whole belt and help utilise the entire surface area available for water to drain off the solids. When solids build up in thicker layers, typically occurring in the centre of the belt, the edges of the feed will dewater effectively, but the increased depth of solids that free water must drain through prevent optimised gravity drainage of water for significant portions of the sludge. The increased

carryover of free water results in increased mass of water requiring removal in the wedge and high pressure sections, and will ultimately result in uneven pressure applied across the pressure sections.

Modifications to the distribution system often do not require major expenditures or additions to BFPs. One site utilised a slotted PVC pipe to distribute the sludge to the GDS. The pipe was only half the width of the belt, meaning about a quarter of the belts width on each side of the distribution pipe saw little to no solids under normal feed rates. The optimisation of this distribution system consisted of replacing the pipe with one that

Table 2. % improvement realised by installing plows into the gravity drainage section of the belt filter pressCake solids

lb solids/t cake

lb H2O/t cake

t cake required to remove 1 t solids

% reduction in t disposed of with 1.8% cake increase

lb H2O shipped to remove 1 t solids

% reduction in lb H2O removed with 1.8% cake increase

12.68% 253.6 1746.4 7.89 13 773

14.49% 289.8 1710.2 6.90 12.49% 11 803 14.31%

Figure 3. Results for System 2.

Figure 2. Results for System 1.

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extended to 2 in. away from the edges of the GDS, resulting in even distribution of sludge feed across the entirety of the belt.

Even with proper and uniform distribution of treated sludge across the GDS of a BFP, water can pool on top of the sludge and not properly drain through the solids now laying on the belt. In order to combat this, many BFPs utilise plows to shift the dewatering sludge to expose new sections of the belt for free water to drain through. In addition to exposing sections of the belt, the shifting action caused by the plows also turns the sludge, creating a path for the free water that pooled on top of the sludge to bypass flowing through the solids and drain much more quickly. Modifications to the plows generally consist of adding additional rows of plows and aligning the plows to get maximum shifting of the solids.

Results obtained through GDS optimisationIn two separate industrial wastewater treatment sites, the GDS of the BFPs on site were optimised. System 1 adjusted the distribution pipe to distribute evenly across the entirety of the belt, as was mentioned previously, but also installed an additional row of plows to accompany the four rows already present. System 2 had a distribution system that evenly distributed the solids across the entirety of the belt, but lacked plows to assist the drainage of free water. In this system, temporary rows of plows were installed on the press. In both instances, no other changes were made to the BFP operation and both systems experienced a 1 – 2% increase in cake solids as soon as the changes were made.

System 1 saw an increase in cake solids percentage from 15.97% to 17.3%. This resulted in a 7.7% decrease in the overall amount of cake shipped off site for disposal and a 9.1% decrease in lb of water shipped off site for disposal with the sludge (Figure 2).

At the average cost to dispose of solids being US$40.925 in the region this site is located, the optimisation of the distribution section cut the price for disposing of 1 t of

biological solids out of the biological treatment area and into landfill by US$19.64.

System 2 saw an increase in cake solids percentage from 12.68% to 14.49%. This resulted in a 12.5% decrease on the overall amount of cake shipped off site for disposal and a 14.3% decrease in lbs of water shipped off site for disposal with the sludge (Figure 3).

At the average cost to dispose of solids being US$40.92 in the region this site is located, the installation of the plows into the GDS cut the price for disposing 1 t of biological solids out of the biological treatment area and into landfill by US$40.51.

ConclusionRising costs of solids disposal across the US demand greater performance and efficiency from the solids dewatering units in wastewater treatment plants in order to control costs. Inexpensive improvements and mechanical optimisations to the GDS result in tens of thousands of dollars in savings by improving the overall performance of BFPs, with little to no costs after the installation and implementation of the upgrades. These improvements pay for themselves in a matter of months and continue to provide benefits for years after installation.

References1. ‘Solids Process Design and Management’, Water Environment

Federation, (2012).2. ANDREOLI, C. V., SPERLING, M. V., and FERNANDES, F., ‘Biological

Wastewater Treatment Series: Volume 6 Sludge Treatment and Disposal’, (2007).

3. TUROVSKIY, I. S. and MATHAI, P. K., ‘Wastewater Sludge Processing’, (2006).

4. ‘Report on the management of wastewater treatment sludge and septage in Vermont: an analysis of the current status and alternatives tio land application’, Conservation, Agency of Natural Resources Department of Environmental, (2015).

5. KANTNER, D. L. and STALEY, B. F., ‘Environmental Research and Education Foundation (2019) “Analysis of MSW Landfill Tipping Fees - April 2019”’, (2019).