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On-Site Wastewater Treatment Using Unsaturated Absorbent Biofilters

Waterloo Centre for Groundwater Research, Univ. of Waterloo, Waterloo, ON, Canada N2L 3G1.

^* Corresponding author.

ABSTRACT

A new type of single-pass aerobic biofilter is being developed as an alternative to the conventional septic tile bed and for treatment of wastewater in general. The Waterloo Biofilter uses absorbent filter media that combine long retention tunes, separate flowpaths for wastewater and air, and large surface areas, thereby enabling loading rates 10 times greater than that for solid particle filter media. Although absorbent sphagnum peat and coarse sand plug readily at loading rates of 50 to 80 cm d^–1, absorbent plastic particles provide consistent treatment with no plugging problems. The latest field trial removes 97.8% BOD₇, 96.1% TSS, and 99.5% fecal coliform bacteria with 12 to 16°C wastewater loaded at 49 cm d^–1. Surge flows up to 204 cm d^–1 over several days are handled with little effect on effluent quality. In laboratory column experiments, removal of fecal coliforms averages >99.99% at 80 cm d^–1 loading, and >99.999% at 10 cm d^–1 after a 10- to 14-d acclimatization period. Ammonium is thoroughly oxidized to NO^–₃ with typically <2.5 mg L^–1 NH⁺₄-N in the effluent. Overall treatment improves with forced air flow compared with natural convection. Cold influent and plugging by freezing are the main causes of poor treatment. A typical household would likely require a biofilter with 3.3-m² surface area in cold climates when flow is balanced, possibly less in warmer areas. This biofilter should also find general application in renovating polluted water, including water for domestic consumption in developing regions of the world.