Municipal tertiary treatment: disc filter with improved pleated configuration

How does disc filtration affect continuous wastewater treatment? Miguel Gutierrez of Siemens Water Technologies explains the basics of disc filtration and how it provides an economical and sustainable solution for municipal tertiary treatment systems.

In disc filters, differences in woven media and flow patterns make each technology unique to a particular application. Given that these units were sold for relatively clean water for municipal tertiary applications, the internal and external configurations are as expected. However, inside-out technologies provide better sustainability and life cycle cost. One of these is the Forty-X disc filter from Siemens Water Technologies, which provides an economical and sustainable solution for municipal tertiary treatment systems.

The first disc filters introduced to the US market used an outside-in flow pattern in which the disc was completely immersed in water, solids were filtered outside the filter media, and the filtrate was collected and transported in a rotating drum and discharged from the system. Accumulated solids are removed by reversing a portion of the filtrate flow through the media shoe by rotating the backwash shoe on the entire drum/disk or by rotating the stationary disk assembly.

The filter media consists of a non-woven structure that relies on a random arrangement of fibers to capture and hold particulate matter. The tissue medium covers the honeycomb structure collected around the filtration drum/pipeline. In this plant, the medium suffers from the inability of the backwash shoe to keep it clean, mainly because it is immersed in an increasing concentration of solids in the feed. As a result, these filters are often complemented by sludge removal systems and additional high-pressure cleaning devices, which adds to the complexity of the overall system.

The main disadvantage of these systems is the biological growth that occurs on the inner surface (filtrate side) of the filter cloth, especially common in non-chlorinated filter feed/secondary effluents. Another problem with high pressure backwashing of these systems is the need to shut down the process to complete the cleaning. Both of these drawbacks result in long flush times, high volumes of waste and excessive requirements to ensure continuous wastewater treatment.

The backwash is initiated by a loss of head and is carried out by high pressure jets which throw the accumulated solids back in the feed direction and are collected in chutes located in the drum/pipe itself. As the disc/drum assembly rotates, the backwash jet is drawn from the same filtered water tank. The medium is a woven polyester fabric with an absolute micron size (typically 10 microns for three reuses). This absolute micron size ensures that particles larger than the holes in the tissue are retained, and particles larger than the existing holes accumulate in the medium.

Both [inside out" and [outside in" technologies are based on a filter media configured as a flat panel. Therefore, the effective filtration area is the usable area perpendicular to the direction of flow and should not take into account the impervious surfaces of load-bearing structures such as frames, panel boxes and steel structures.

The inside-out disc filter gives the filter panels a pleated shape, increasing the effective filter area by 40% compared to a flat design. It also increases the overall strength of the filter plate, allowing it to withstand higher operating head losses. The corrugated structure makes the filter plate less prone to deformation of the fabric medium.

The combination of a larger filter surface area and a more robust disc filter configuration results in high throughput. In addition, the filter plate seal is hydraulically boosted, improving seal reliability and eliminating many of the leakage points inherent in the plate design. In addition to the design of the filter plate, the filter box (the outer shell of the plate) is designed to contain debris. Existing flat panel technologies have internal gussets in their filters that provide a surface for large inorganic materials such as plastic, algae ropes, rags, and other floating materials that can hang, build up, and interfere with the internal hydraulics of the filter panels. The disc filter box does not have these internal folds, so this unwanted material can freely and continuously flow and be removed from the system.

At the pollution control plant in Thomasville, Georgia, USA, the 40-X disc filter was tested on the production plant for several months. The installation includes jet filtration cleaning followed by bubbling aeration of secondary effluents. This test is intended to check the performance of the device and determine the operating parameters. Part of the test included peak feed solids conditions. On fig. 1 shows typical trends in Total Suspended Solids (TSS) from integrated testing.

Regardless of the feedstock concentration, the effluent TSS was consistently low due to the limited opening of the woven fabric. The curve was drawn at a hydraulic load of 6 gpm/sq. foot, true effective filter area and 50% immersion. As expected, the higher the solids content, the more backwash required to keep the medium clean. On fig. 2 shows a low reject rate as a function of turbidity. The filters provide very low rejection, typically 1.5% or less. The annual cost of automated operation of backwash pumps associated with clouding phenomena is also lower.

The institution needs to expand its capacity to cope with growth and the new consent ordinance. Unfortunately, the lack of available land prevented the addition of more concrete axial filters to the factory design. Therefore, the existing mobile bridge filter was replaced with a 40X disc filter (see Fig. 3). The unit is able to accommodate two 40X disc filters in half the footprint of the mobile filter to increase the capacity of the unit. In addition, the pleated filter plate configuration allows for up to 40% more filtration area with fewer disc filters. This leaves room for additional disks to be added to meet future flow requirements without changing the design or filtration system.

To meet growing infrastructure needs, Kuwait has planned the construction of a state-of-the-art wastewater treatment plant with a capacity of 72 million gallons per day. The specifiers chose disc filters because they recognized the value of pleated filter panels in this high flow processing application. The filtration area is increased by 40% compared to the flat panel design, which reduces the need for single processes, resulting in savings in footprint, civil works and overall installation costs. In addition to disc filters, Siemens also offers bioprocess solutions, including fine bubble vertical loop reactors and purifiers.

The problem with many infrastructure renovation projects in very densely populated areas is the lack of available space. This was the case with the modernization of Wuxi, one of China`s largest wastewater treatment plants. Since there was no room for a filtration step, the facility decided to upgrade the existing concrete contact chamber with chlorine disc filters and ultraviolet (UV) disinfection (see Figure 5). The shallow profile of the disc filter, combined with the 600 square meters (12 34 sq ft) surface area available in a pleated filter configuration, can increase the capacity of an existing concrete chlorine pool.

The droughts of recent years, combined with an increased demand for drinking water for irrigation, have forced many regions of Italy, especially in the agriculturally rich south, to focus on the reuse of wastewater. In the near future, it is planned to launch more than a dozen disk filters for several communities and service providers.

The Batch Sequencing Reactor (SBR) plant was in dire need of improved total suspended solids (TSS) removal without a properly functioning slow sand filter. The solution is a plug and play installation consisting of a disc filter placed on top of an existing SBR surge tank (see Figure 6). The small footprint of the filter saves the plant from additional construction work, all within the plant`s hydraulic configuration, including existing UV disinfection equipment.