Anaerobic digesters

Anaerobic bacteria -- microbes that live without oxygen -- are helping to turn plant and animal waste into energy and to clean up waste discharges.

Although the concept of burning biogas released from waste products has been around for centuries, waste producers in the United States have only recently begun to use the technique.

Currently, there are about a dozen major anaerobic digesters producing biogas on a commercial basis. This gas is about 60 percent methane and 40 percent carbon dioxide. It can be compressed for storage or used immediately to produce energy on site.

In most cases, operators finance their systems by selling electricity back to their local power companies, as well as by their own energy costs. Energy use on site often involves ''cogeneration,'' in which waste heat produced by the biogas process is also used as an energy source.

Ancient civilizations used naturally occurring biogas for cooking. Today, most such gas is produced outside the US. In China, for instance, there are an estimated 7 million family-size digesters, plus 560 rural power stations fueled by animal and plant waste. While some municipal waste water systems generate methane in the US, the inclusion of highly toxic industrial wastes usually confines that production to the later stages of effluent treatment.

Meanwhile, the private commercial digesters already ''on stream'' are working examples of how the entire waste product can be used. Besides producing energy and nutritious food supplements, the systems eliminate most environmental pollutants, greatly reduce odor, and speed the conversion of waste-treatment settlement lagoons into a reusable water supply.

Mr. Umstadter has worked to help set up four systems in the US and Guatemala for dairy and livestock waste. He says he considers that the lush tropical areas of Latin America have great potential for producing energy from fruit and vegetable waste.

Turning a manure processing system into a plant digester requires little alteration. It is more a matter of ''materials handling,'' he explains. Recently Agway, a leading agricultural cooperative in the Northeast, agreed to market the AES systems through their outlets.

Mr. Umstadter had been active as project manager in helping nearby Kaplan Industries install what now is the country's largest methane digester at its cattle feedlot and 250,000-head-a-year slaughterhouse. The project, funded largely with a $920,000 grant from what is now the Department of Energy, was completed in 1979. The digester can process 25 tons of manure a day. This generates $158,000 worth of methane and $380,000 worth of nutrient-rich byproducts annually.

Before it had the methane digester and anaerobic-aerobic lagoon system, Kaplan had been discharging one-quarter- to one-half-million gallons of waste water daily from its processing plant and feedlot. Kaplan officials had installed an earlier lagoon system themselves. They modeled it after similar settlement lagoons used throughout Europe.

Seven empty phosphate pits, left behind by strip-miners, provided a pond system with the help of separating dikes. The first several anaerobic ponds in the 100-acre system held nutrient-rich solids, which settled out there. Succeeding ponds were oxygen-supporting (aerobic) bodies of water. The first, deeper ponds promoted a natural anaerobic conversion of the methane - which was then lost in the atmosphere - while the subsequent, shallower ponds promoted the cleansing, aerobic action to make the water reusable on-site.

Although recyclable, water in the latter ponds was still so nutrient-rich it encouraged heavy algae growth, which interfered with reuse. A fast-growing, freshwater tropical food fish, the tilapia, was then introduced to control the algae. The tilapia, a weed-eating perch, was so successful that Kaplan hired commercial fishermen to harvest the fish three times a year for use as a frozen white fish product.

The anaerobic digestion system was then added to take the pressure of settling solid wastes off the lagoon system and to produce energy.

According to AES vice-president Elizabeth Coppinger, the system works like this:

* An automated scrapper periodically removes manure which falls through concrete slats in the floors of the six quarter-mile-long sheds housing the cattle.

* The material is mixed with waste water with organic residue and ''pouch manure'' (undigested waste) from the processing plant and then fed into the two 320,000-gallon digester containers.

* As the anaerobic bacteria breaks down the volatile solids, the biogas is released through the top of the containers. New waste continually enters the container, spending an average of 15 days inside before the fermentation is complete.

* The biogas is siphoned off to fuel the company's steam-producing boilers used in meatpacking, and on weekends and holidays (when the boilers are inactive) it fires a 440-kilowatt generator. The generator produces electricity to be channeled into the grid of local Florida Power Company, which is required by federal regulations to pay Kaplan for the energy.

* The digested solids are pumped from the containers as an inert and odorless material with an almost soil-like consistency. It can also be used as a feed supplement for livestock. In other forms, it can be added to building materials, horse feed supplements, and even used as artificial fireplace logs.

* The nutrient-rich liquid from the digester then enters the first of the anaerobic lagoons, creating tiny methane bubbles from the rest of the undigested waste. (At this stage, some farmers simply use the liquid for irrigation.) ''The lagoons are set up to confine waste and to allow the breakdown to occur on the property,'' Ms. Coppinger explains. ''The breakdown will occur anyway. But it will happen somewhere down the road in a stream - creating environmental damage.''

Since the Kaplan installation, AES has created a more simplified, modular digester system designed for use on a ''turnkey'' basis. The systems feature six sizes of digester containers, a gas collection system, and a fail-safe control panel to monitor the operation. To make the system economically feasible, an owner should have at least 200 dairy cows, 1,000 beef cattle, 400 swine, or 50, 000 laying hens, according to Mr. Umstadter.

AES currently operates a working model of its system on a 100-head dairy farm in Bedford, Va., to introduce the system to the general public as well as to prospective clients. But Mr. Umstadter figures that the average American dairy farm is probably closer to a 600-head capacity. ''Generally speaking, at today's rates - and taking advantage of energy investment tax breaks - a farmer can put in an anaerobic digester system that pays for itself in five-six years,'' he says.