In this article Stuart Christian, Director of Greener Waste Limited explains how a simple, low cost process can dramatically improve the performance of failing filter beds.

Filter beds are a simple and effective way of reducing the organic loading within wastewater in a carbon footprint friendly manner. A simple filter bed is made up of a media layer, for instance broken blast furnace slag, laid on top of drainage tiles to a depth of approximately 1.75 metres, the whole of which sits on a concrete base with a central drainage channel leading to a collection sump.

Media size is an important element in the treatment process to ensure an adequate supply of oxygen. The gap between the media must be sufficient to allow water to pass down while air and therefore oxygen pass up. Wastewater is continually spread over the top of the bed by a distribution mechanism, allowed to run down through the bed and out through the bottom outlet channel. As the wastewater passes over the media, oxygen in the air enables bacteria to grow and form a film on the media. It is the bacteria that digest the organic matter within the wastewater.

When correctly loaded, the filter beds reduce ammonia through a process known as nitrification. The filter beds have two zones, the first is at the top of the bed where carbonaceous bacteria feed on the BOD element of the contaminated water and the second zone is in the bottom third of the bed where nitrifying bacteria reduce the ammonia.

In the nitrification process it is important that the BOD is reduced efficiently first so that the nitrifiers, which are slow growers, can exist and perform comfortably. If the equilibrium of the bed is upset, the performance of the nitrifiers can be quickly diminished. For example, blockage of the bed by a build up of excess film can dramatically reduce oxygen transfer. The carbonaceous bacteria have to move further down into the bed to gain access to more oxygen and, as a consequence, out-compete the nitrifiers. Media blinding is a common problem experienced by most beds at some stage in their life and in the past the only way of remedying the problem has been to dig out the media, wash it and then replace it at significant cost and disruption to the site.

However, there is now a simple, low cost process that can dramatically improve the performance of failing filter beds. This bio-remediation process, designed and developed by Greener Waste Limited, enables failing beds to be brought under control and made to operate efficiently without recourse to digging out old media with all of the associated problems and costs. The Greener Waste Limited process also works efficiently on plastic media filters and with dramatic results.

In a recent example, Greener Waste Limited was asked to look at a sewage works where, despite the best efforts of the utility staff, the site was constantly failing its consent limits on ammonia and BOD. The filter beds struggled to nitrify. A large plastic filter had been retrofitted with the aim of reducing the load prior to the granite filter beds, but refused to work at all.

The problem was caused by a food processing factory that sends high volumes of oil and fat down to the works on a regular basis. The works became clogged with the FOG (fat, oil and grease) resulting in thick blankets of solid, putrefying FOG floating on the surface of the PSTs and pumping station. The works has a pe of 6,000, with the processing plant contributing 3,000 pe, and consists of screens, primary settlement tanks, two granite filter beds, a humus tank and, of course, the retrofitted plastic filter.

The most effective way of bringing the site performance under control was to enable the works to remove the FOG problem and allow the filters to degrade the BOD load more quickly and efficiently within the correct zone of the bed. Bio-Selector technology was installed at the head of the works and carbonaceous bacteria dosed into the primary settlement tank. FOG was degraded prior to the plastic filter and the oily film that coated the plastic media was replaced by the correct biological film.

After four weeks the fat cap on the PSTs and pumping station had dramatically reduced in size. Fat carry-over to the filter beds was eliminated, and the plastic filter was reducing COD and ammonia to below discharge consent levels. The overall performance of the site had now improved to its original design specifications. In addition, the plastic filter now works so well that it will be possible to eliminate the original filter beds with further savings being made in reduced pumping costs and maintenance. This has all been achieved at a cost far below that of any suitable alternative capital solution. Other works that do not have FOG problems, but suffer from poor nitrification, have also demonstrated significant improvements from the Bio-Selector process.