Dual Stage Biofilter for Publicly Owned Treatment Works

This page updated November 18, 2005.

The Reynolds Group

Dual Stage Biofilter for Publicly Owned Treatment Works

CARB Grant Number 96-339

The statements and conclusions in this Report are those of the grantee and not necessarily those of the California Air Resources Board. The mention of commercial products, their source, or their use in connection with material reported herein is not to be construed as actual or implied endorsement of such products.

Foul and odorous air emissions from wastewater plants, commonly known as Publicly Owned Treatment Works (POTW's), pump stations, and other industrial sources such as pulp and paper mills and refineries are common nuisances. The ingredients of the effluent air streams may contain a variety of volatile organic compounds (VOCs) that are subject to air pollution regulations. One of the key components of the odors is hydrogen sulfide gas (H2S). Hydrogen sulfide is easily oxidized into sulfuric acid, but sulfuric acid creates havoc for conventional treatment methods. Sulfuric acids corrode thermal oxidation systems (conventional burner systems) and require high capital costs when treated chemically (typically using chlorine).
Biological treatment of air pollutants is appealing because biofilters are a benign and environmentally friendly technology. The primary barrier to adoption of biofilter technology at POTW's and other applications is the lack of operating experience. Plant managers are understandably reluctant to adopt new systems without a successful track record. Even the California Air Resources Board (CARB) was initially hesitant to support a system that did not work on traditional mechanical or chemical principles. This project was the first biologically based system that the CARB funded.
Under CARB sponsorship, The Reynolds Group, the University of Southern California and the Ojai Valley Sanitary District teamed to demonstrate that a two-stage biofiltration system can effectively treat H2S. Conventional single-stage biofilters are subject to the accumulation of acids and early death.
Air pollution control (APC) by biofiltration is accomplished by passing contaminated air through a damp, porous medium that supports a vigorous mixed culture of microorganisms. The contaminants are biodegrade. Compost is commonly used as a support medium, although synthetic media and a variety of mixed media re also used.
Biofilters are an excellent technology for treating air emissions at POTW's if the difficulties caused by H2S can be solved. Current biofilters at POTW's operate as single-stage open reactors. A single medium, usually compost, is expected to treat all of the compounds that must be removed. Some alkalinity may be added in the form of calcium carbonate minerals, but this is easily exhausted by sulfuric acid production. The spent minerals and acid-degraded compost often form small particles that contribute to biofilter clogging. The compost has a very limited lifetime.
The POTW biofilter for this project contained two stages. The first stage was an enclosed system with a lava rock support medium. The first stage was optimized for removal of H2S and utilized acidophilic autotrophic bacteria such as Thibacillus thiooxidans. These strains are adapted to sulfide oxidation as an energy source, and survive well at low pH. This microbial ecosystem is relatively well known because it is responsible for the sulfuric acid corrosion of sewer piping. Because H2S removal was confined to the first stage, there was no acid or inorganic sulfur production in the second stage compost bed. The acid produced in the carbon bed was drained and returned to the swage flow. Only a small volume of liquid required handling.
Some oxidation of VOCs occurred in the lava rock bed. In this project, the first stage biofilter was sized only to remove the H2S. This minimized initial costs and provided a dual phase system with maximum versatility.
This report summarizes the year-long full-scale demonstration of the two-stage biofilter at the Ojai Valley Sanitary District's newly modernized POTW. Topics presented in this report include a description of the demonstration, the sampling methods, and the results. The conclusions of this project are that:
  1. A lava rock pre-stage biofilter can be a very effective means of removing H2S and VOCs from the waste air flow stream at a POTW or other facility where H2S is a component of the effluent.

  2. The low-pH biofilter can effectively treat H2S emissions at contact times as low as 12 seconds, which means that the pre-treatment unit can be very small and that current compost-based biofilters may be over designed.

  3. Lowering the pH in the first stage of the biofilter neutralized the pH in the second stage of the biofilter resulting in longer life of the second bed.

  4. Lava rock provides an excellent medium for a low pH biofilter.

  5. VOCs can be removed in the low pH biofilter.

  6. Internet based instrumentation and software that can monitor and control the performance of the biofilter from remote locations have serious implications for the business model that will successfully compete in the industry, and

  7. Traditional problems associated with biofilters including acidification, flow heterogeneity, and measurement can be resolved with proper design and sufficient operating experience.
The environmental benefit of biofilters will be the improved treatment of POTW discharges, reduced odors, reduced neighborhood exposure to toxic chemicals, and the reduced generation of smog precursors. Economic benefits will accrue first to sanitary districts in California, which will have an inexpensive means of meeting their regulatory responsibilities. Industry will benefit as more installations are made, providing employment for engineers, contractors, and operators.
The primary barrier to adoption of biofilter technology at treatment plants, and particularly advanced technology, has been the lack of operating experience and dissemination of information regarding the use of biofilters at POTW's for the treatment of H2S emissions. Plant managers are understandably reluctant to adopt new systems without a successful track record. This project with the support of the CARB directly addresses this problem.

Funding Source

Funding Amount




$  57,418

Ojai Valley Sanitary District

$  75,000

Univesity of Southern California

$  10,000

 Click here for the entire final report.

ICAT Funded Projects