wastewater treatment plant
Biofiltration – biofilter – odor control and emission treatment
A biofilter is one of the most economic and simplest odour control systems available. For a good functioning biofilter however, several conditions are to be fullfilled.
Biofilter operating principle
For biofilter applications, the filling material or biomass is composed of organic material such as bark chips, compost, peat, heath, coco, root wood, expanded clay granules, etc. in a well defined mixing ratio, or in several layers. This natural “filter bed” or biomass can be contained in an open or closed biofilter and is surrounded by a thin water film. Part of the pollutants in the emission flow directed through the biomass will already dissolve in this water film; part of the pollutants is retained by the biomass particles and the remaining pollutants will be further decomposed by the microorganisms present in the biomass. These microorganisms will collaborate to decompose the remaining pollutants out of the emission flow. The microorganisms partly feed themselves with the remaining pollutants, partly with the biomass itself. Water and oxygen are of vital importance for the microorganisms; both these substances are, in normal operating circumstances, present in the biomass. The residual of these decomposition processes are CO2, H2O, sulfate, nitrate and similar. As the biomass gets saturated with pollutants and residuals and gets crumbled by the action of the microorganisms, it is to be replaced periodically. The frequence of these biomass’ replacements is depending on the emission flows offered and the charges present in the waste gasses. Not all emission flows can be treated in a biofilter. The pollutants present in the emission flows will determine wich bacterial groups will survive and/or be predominant in the biomass (at the same time, under the influence of temperature and moisture content, a certain microclimate and balance will develop in the biomass). Every change in these factors can severely destabilize the efficiency of the biofilter.
Biofiltration : important parameters for an efficient biofilter
Constant moisture content: Preservation of a optimal moisture content in the biomass is an essential criterion for a good functioning biofilter. The biomass should be neither too dry, neither too wet. Too much humidity can make the biomass clot together, the air resistance will increase, the oxygen level will decrease, the filter itself will become a source of odor nuisance. Therefore, it is of utmost importance to prehumidify the emission flow, prior to direct it through the biofilter. This prehumidification unit has to be perfectly adjusted in order to create the correct moisture content within the biofilter.
Constant air supply(emission flow and oxygen level) : There should be a relatively constant emission flow. When the emissions can only be produced at a rate of 7 to 8 hours/day during 5 days/week, with weekend breaks and short and longer holiday stops, a biofilter will not be a good solution. The microbial growth cannot survive without permanent oxygen supply. The emission flow should also be free of dust and grease.
Constant temperature:(ambient temperature) : for a good development of the microbial growth a temperature of 15 to 20 ° to max 30 °C is ideal. Lower temperatures will slow the process and higher temperatures will cause of proliferation of bacteria, that will destabilize the entire biomass.
Waste load as constant as possible : A lower pollutant load isn’t a problem as such, and will only transcribe into a higher elimination result (only on condition that the other parameters remain at a constant level). At a high or peak pollutant load, the biomass will clot together at an accelerated speed, because of the sudden increase in microbial growth.
No high concentrations of toxic and/or acidifying components: Very toxic substances will kill the bacteria. Too high concentrations of sulphurous, chlorous and nitrogenous organic components as well as ammonia will eventually acidify the biomass and decrease the its efficiency. This can partly be prevented by adding certain additives to the biomass, by replacing the biomass more frequently or by installing an upstream chemical gas scrubber.
Custom-made biomass: The biomass should be entirely adapted to the emission flow offered. The biomass shouldn’t be too fine nor too coarse, the air should be evenly distributed over the entire biofilter. The biomass should be coarse enough to allow a sufficiently large exchange surface, but should also contain enough organic material for a satisfactory microbial growth and a good living environment for the bacteria. Additives such as lime and dolomite can prevent a premature acidification. The biomass is preferrably installed on a grid, to improve the air distribution. For special applications, it might be appropriate to used an inoculated biomass (inoculation with specific bacteria depinding on the kind of application). In all cases, the biomass should be periodically replaced, in function of the acidification and the salification of the biomass and the pressure drop over the biofilter (because of the compaction of the biomass). The lifetime of the biomass is significantly varying and is to be set between 6 months and 5 years.
- Low investment cost (simple construction)
- Good result for biologically decomposable components in relatively low concentrations (polishing technique)
- Takes a relatively large surface
- Biomass needs to be replaced periodically
- Exact moisture content and pH need to be monitored accurately, a continuous aeration is vital
Biofiltration : most important application areas
- Wastewater treatment plants
- Agro-industry (composting, food industry, meat processing and fish processing industry)