Anaerobic conditions in sewers and septic tanks result in the formation of hydrogen sulfide which causes various problems, among others odors. An anaerobic environment also results in methane formation. Methane is a powerful greenhouse gas which has an influence on global warming.
This project examines two Hybrid Filter A/S filter products, Hybrid Brøndkarmsfilter intended for drain shafts and Hybrid Mini Kulfilter intended for septic tanks, both based on activated carbon. The purpose of these products is to remove odor from sewers and septic tanks respec-tively.
The capacity and efficiency of Hybrid Brøndkarmsfilter to remove hydrogen sulfide is investigat-ed under controlled laboratory conditions and under field conditions in drain shafts. The filters are analyzed for water content, pH and sulfur compounds after use in hydrogen sulfide removal experiments. Air exchange from the sewer is examined, based on measurements of differential pressure across the filter, and hydrogen sulfide concentrations to determine filter load. Several attempts to improve the filter are tested; including regeneration of a used filter, reduction of the filter load by expanding the filter insert with additional chambers, and investigation of whether the filter is loaded evenly. The ability of the Hybrid Brøndkarmsfilter to adsorb me-thane is investigated under controlled laboratory conditions.
Hydrogen sulfide and methane concentrations, as well as air exchange from two septic tanks are examined to determine the load of the Hybrid Mini Kulfilter. The ability of the filter materi-al to adsorb and degrade methane is also investigated.
The studies of capacity and efficiency show that Hybrid Brøndkarmsfiltre have a capacity be-tween 10.3 and 149 mg H2S/g activated carbon before breakthrough of hydrogen sulfide.
Analysis of used filters shows a very low pH in the water content of the filters, and that the sulfur content in the filters almost entirely consists of sulphate, indicating biodegradation of hydrogen sulfide in the filters.
The average air exchange from drain shafts through Hybrid Brøndkarmsfilter is between 11.3 and 21.4 L/min and -26.7 to -37.6 L/min. The load of hydrogen sulfide from the drain shaft with the highest load is 1.17 g/day and a Hybrid Brøndkarmsfilters breakthrough capacity will be exhausted after a period of 4 to 54 days.
Studies of possible improvements of a Hybrid Brøndkarmsfilter show that it is possible to re-generate a used carbon filter by rinsing with 417 L of water / kg activated carbon. The regenera-tion raised the efficiency of the filter from 12 % to 50 - 60 %. By expanding the filter insert with two additional chambers the filter load of hydrogen sulfide can be reduced by 58 %. Studies of air velocities above a filter show an uneven distribution, and the filter capacity is therefore not fully utilized. A potential for methane removal in an unused Hybrid Brøndkarmsfilter has been observed, as the filter adsorbs methane.
Hydrogen sulfide concentrations are measured between 0 and 27 PPM in septic tanks, however no hydrogen sulfide is measured below the Hybrid Mini Kulfilter. The measured methane con-centration in the aqueous phase ranges from 0.82 to 1.86 mmol/L. Methane concentrations, below Hybrid Mini Kulfilter, have been measured between 0.02 and 0.05 mmol/L. The air ex-change from the septic tanks has not been measured successfully, and therefore the filter load cannot be determined.
Unused filter material from Hybrid Mini Kulfiltre can adsorb relatively small amounts of me-thane. Whereas used filter material from Hybrid Mini Kulfitre does not adsorb methane. No biological degradation of methane is observed in the filter material.