German research underway on trickling filter practices
Trickling filter technology is one focus of a joint research project – EXPOVAL – funded by the German government that will validate engineering guidelines and optimum conditions for achieving high performance. Dr. Christian-Dominik Henrich of GEA 2H Water Technologies Gmbh reports.
Poor wastewater treatment is one of the biggest enemies of a safe and sustainable water supply all over the world. Next to frugal handling of existing water resources, the treatment of wastewater, sometimes towards future reuse, is important. Unfortunately, the elusive effects of improving water supply through groundwater recharge or surface water must be weighed against capital cost, cost of energy demand, and other variable costs of wastewater treatment facilities. With the costs per unit of energy constantly rising, it is of utmost importance that future wastewater treatment is energy-efficient. Important considerations, especially in developing countries, should include the reliability and simplicity of a wastewater treatment process. I.ow-maintcnancc unit operations ensure a continuous treatment of incoming wastewater.
A trickling filter is a fixed-growth biofilm treatment system where the wastewater trickles through a media on which a biofilm grows. The wastewater is distributed at the top of the filter, with the use of rotating distributor arms that can be either hydraulically or electrically driven- Oxygen is provided to the system through ventilation openings at the bottom of the filter; through which air can freely (low. The media is placed onto a substructure usually made out of parallel beams placed on concrete feet- Originally, trickling filters were packed with a stone or lava filling. Now, these filters are packed with plastic media - first made out of thermoformed polyvinyl chloride (PVC) sheets glued to structured packings, and later made out of the more environmentally friendly polypropylene corrugated sheets that are not glued but welded together to structured packings.
Until the 1980s, trickling filters were promoted in the western hemisphere as an energy-efficient process for using microbial systems to treat wastewaters. In those areas, the only need for energy was to lift effluent by pumping it for distribution on top of the filter. Pumping costs were reduced by using hillsides for gravitational How. I hi- iiKiin difference between trickling filters and an activated sludge system is in oxygen supply. In trickling filters, oxygen demand is often satisfied by natural ventilation without any need for energy-intensive aeration and high-tech equipment- However, increasing effluent demands and process issues - such as media clogging in conventional, stone-packed trickling filters, along with a poor understanding of nutrient removal characteristics and poor modeling of actual processes inside trickling filters - led to a decline in the use of trickling filters.
Because of the newly developed plastic media, as well as the increased importance of sustainability, the trickling filter is going through a renaissance. Especially in combination with other unit operations such as anaerobic pre-treatment and optimized process design, these new-generation trickling filter systems are able to treat wastewater to very high standards, while offering low energy demand and a high degree of simplicity and robustness. Because of these improvements new trickling filter projects arc being implemented all over the world, many of them receiving funding from institutions such as the KfW Development Bank in Germany, which focuses on sustainable development.
With the new generation trickling filter technology and an intelligent plant design and operation, high flexibility is achievable. ITiis includes the abilitv to treat waters to effluent quality comparable to activated sludge (AS) and activated sludge biological nutrient removal (AS-BNR) processes. Additionally, these filters can provide the ability to produce a variety of effluents that arc treated to meet specific local needs during seasonal variations at very low operational and maintenance costs.
Unfortunately, existing engineering guidelines are primarily based on research that was done long ago, when stone-filled trickling filters were standard. Here, many operational recommendations and design equations or constants cannot be directly transferred to new generation trickling filters filled with structured packings. Additionally, these guidelines were developed for standard western climate zones at moderate temperatures. Hence, when using these old design equations for filters located in hot or cold climates, these filters are vastly over-or under-designed.