Drum filter or disc filter?
Drum filter or disc filter?——Experiences from Aquasust facilities and tests
One of the key treatment steps in recirculating aquaculture system plants is efficient and gentle particle removal. It is important to get the particles out as quickly as possible so that they are not degraded into finer particles or broken down by bacteria in the water.This addresses some of the differences between using a drum filter and a disc filter in a Aquasust context.
Although the drum filter is one of the most widely used particle removal technologies in RAS today, there is another well-prover microscreen that may have advantages especially in the RAS system of the future.This is the disc filter.
How drum filters and disc filters work
As the name indicates, the drum filter is built around adrum shaped skeleton enclosed with external filter panels.The untreated water flows into the drum, and gravity drivesthe water through the filter panels and out of the drum.In the disc filter,the influent water also enters acentral drum.The drum has a number of openings thatdistributes the water to the filter discs . The disc segmentsare made up of several filter cassettes with filter cloth onboth sides.This increases the filter area considerably.
The water is filtered passively through the filter cloth in a similar manner as for the drum filter. However, the disc filter has a significantly larger filter area which provides a more efficient filtration, or alternatively a smaller footprint. As particles accumulate on the filter cloth and the resistance
increases, the water level inside the filter will rise with time.Common to both types of filters is that a backwash cycle is automatically initiated when the water level reaches a certain level. The filter panels are then sprayed from the outside to clean the filter cloth. Reject water containing particles scollected in a long trough located inside the filter,along which it exits, e.g. to a collection tank for further sludge treatment.
Larger filter area in a smaller space
A disc filter has more than three times the filter area of a drum filter with a corresponding footprint. It is therefore possible to use much finer cloth in the disc filter and still ensure favourable overcapacity in relation to the filter area.This prevents the differential pressure. from increasing too much over time as a result of fouling of the mesh. For example,it is possible to use 30 to 50-micron cloth instead of 60-micron whichi s widely used in drum filters. In general, fewer and smaller units of disc filters will be needed when compared to drum filters. Being able to take up less space for filter installations can also have other positive economic aspects,for example by reducing the size of the building and the plant site.
Backwash water and energy consumption
Tests with water have shown that a disc filter consumes 45% less backwash water than a drum filter while extracting the same amount of suspended matter. The tests were performed in parallel with water from the same fish tank.The reject water was collected, and the sludge volume was measured after sedimentation. Following a proper mixing of the sediment, a sample was taken to measure the concentration of suspended solids (SS). The trial was repeated 10 times. The result showed that the SS concentration in this experiment was almost twice as high in the reject water from the disc filter compared to the drum filter. So, what are the benefits of being able to minimise the amount of reject water beyond reducing the water consumption?
Firstly, the operating cost is reduced corresponding to the reduction of the backwash water.The energy consumption - which is mainly spent on pumping backwash water and rotating the drum-is therefore 45% lower in a disc filter than in a drum filter.
Secondly, there will be less volume of sludge that must be stored and dewatered. This in turn can reduce the need to invest in sludge storage and dewatering equipment.
