Getting Moisture Into The Compost Pile

From BioCycle
Journal of Composting &Organics Recycling
June 2001, Page 51

GETTING MOISTURE INTO THE COMPOST PILE
A survey of operators in various regions of the U.S. provides insight into keeping the composting process wet enough — the what, when, how, and how much of adding moisture.
Robert Rynk

Composting is a thirsty process. To sustain the biological activity that converts raw feedstocks into compost, a healthy proportion of water is required (more than half by weight). Initially, the combination of feedstocks, and if necessary added water, provides sufficient moisture to start the process. Afterwards, a large amount of water is lost by evaporation due to the heat generated, air movement, turning, and exposure at the pile surface. Because water is essential, it must be replaced as it is lost. If not, the process will slow and eventually grind to a halt. One option is to let the process slow and wait for the next soaking rain — an unreliable tactic. Therefore, most operators in dry climates, and many in wet climates during dry periods, add water to composting piles, windrows and vessels.

Although it may surprise some composters in northern climates struggling with too much moisture, keeping windrows and piles moist enough is one of the biggest challenges that many compost operators face. Both adding enough water and getting it thoroughly distributed are difficult. Maintaining adequate moisture adds considerably to the labor and equipment costs of composting. Operators have adapted a variety of procedures, equipment, and moisture sources for adding water. BioCycle recently surveyed several compost facility operators to help determine what practices are prevalent for getting moisture into the pile.

Sources of Moisture

Sources of moisture used by composting facilities include conventional groundwater and surface water supplies, runoff storage ponds, manure storage lagoons, treated wastewater, wet feedstocks, and other readily available water sources such as condensate traps. Because composting facilities tend to use a huge volume of water, sources must be at least inexpensive and preferably free. Even better, tip fees may be associated with some moisture sources, such as liquid food residuals.

As Table 1 shows, the facilities that we surveyed use a variety of water sources. Most of these operations prefer recycled water. The Land Recovery, Inc. (LRI) facility, an aerated and turned pile composting system in Puyallup, Washington, uses condensate captured from the forced aeration system. The water is stored in below ground tanks, which also collect leachate from the biofilter. Fresh water is used only for a mature pile that needs moisture. Earthworks, which manages several on-farm composting operations in New York State, adds liquid cattle manure or milking system washwater when windrows require water. At one site, water is pumped from a nearby creek when manure and wastewater are not available. Farm composting operations typically avoid using fresh water because many farms have ponds for holding liquid manure, milk processing water and site runoff. The biosolids composting operation in Carlsbad, New Mexico uses treated wastewater obtained from the adjacent wastewater treatment plant. The recycled water is disinfected at the treatment plant before being distributed for reuse at the composting facility (and for irrigating parks and golf courses). The Intervale Composting facility (Burlington, Vermont) and the Texas Organic Products (TOP) operation (Austin, Texas) each obtain wet feedstocks that serve as moisture sources. In Intervale’s case, processing residuals are obtained primarily from the Ben & Jerry’s ice cream plant. The TOP facility obtains a variety of wet residuals including milk processing water, out-of-date beverages, and other liquid food processing residuals from local suppliers (see “Collecting and Processing Liquids and Other Difficult Feedstocks,” November, 2000). In addition, both facilities collect water from site runoff and reapply it to their windrows. However, site runoff makes up a relatively small percentage of the total amount of moisture added (approximately 10 to 20 percent).

Weed seeds and/or pathogens are a consideration for some sources of recycled water, including liquid manure, some food processing residuals, water from storage ponds and lagoons, and untreated wastewater. If weed seeds and pathogens are a particular concern, prudence suggests that these water sources should not be added after temperatures fall (below 131°F, for lack of better guidance). Regulations may determine how and when certain sources of recycled water can be added. If moisture is needed after temperatures subside, a water source that is not a pathogen risk should be used. For this reason, the LRI facility uses only fresh water for mature windrows that require moisture (i.e. have already attained PFRP compliance). Fortunately, mature windrows are normally managed at a lower moisture content so fresh water is a relatively small part of the total.

Adding Water – How Much and When

According to the books, composting feedstocks should be maintained at a moisture content in the range of 40 to 70 percent (preferably 50 to 60 percent) to keep the process active. Our survey suggests that compost facility operators do try to maintain moisture in the 50 to 60 percent range. However, operators in dry regions tend to aim for a lower range, commonly 45 to 50 percent. (other things being equal, it takes one-third less water to maintain a range of 40 to 50 percent compared to 50 to 60 percent.) In most cases, operators in all regions allow the moisture content to fall as the compost reaches maturity in order to facilitate screening and product consistency. Target moisture contents at this stage fall in the range of 30 to 45 percent.

The amount of water needed to maintain the desired moisture content obviously varies with operational and environmental factors including the feedstocks, ambient temperature, precipitation, wind, exposure, stage of composting (e.g. heat generation), air flow, agitation, porosity and moisture conservation practices in place (see “Conserving Compost Moisture” October, 2000). Higher ambient temperatures, less precipitation, more wind, greater exposure, more process heat, greater aeration, and more agitation all increase moisture loss and therefore the need for added water. In practice, facilities tend to water more liberally if the method of adding water is convenient and the water itself costs little.

Estimates for the amount of water used by several facilities are given in Table 1. Based solely on this information, it is difficult to generalize about the amount of water needed. The feedstocks and water sources appear to make a difference. For example, the Intervale facility, located in the cool and moist climate of Vermont, uses approximately 100 gallons of liquid/cubic yard (cy) of compost. In comparison, the Carlsbad composting operation in arid New Mexico estimates water use to be 20 to 60 gallons/cy. The difference is largely due to the source and availability of moisture. Intervale’s estimate of moisture use is determined by the food processing liquids that it accepts as a primary feedstock. They take whatever amount is delivered and occasionally need to find dry feedstock to balance the moisture. In addition, the Carlsbad facility uses less water, in part, because they manage composting at a lower moisture content.

The water use estimates at the TOP operation provide an indication of seasonal water needs. During the hot dry weather in July and August, they can add 6,000 gallons of liquid to each 800-cy windrow daily. In comparison, during cooler periods they would typically use the same amount over a week. These estimates are high for most facilities because of TOP’s climate and its access to liquid feedstocks. As a gross generalization, over 200 gallons of water/cy of compost produced may be needed in hot arid climates during the warm season and about half that during the cool season. Where rainfall and temperatures are moderate, for example during the summer in the Northeast, a batch of compost may require 50 to 100 gallons of water/cy.

The timing and frequency of adding water depends on the same factors. Many operators monitor pile moisture content and add water when the average moisture content falls to some lower limit, typically between 40 and 50 percent. Although measurements can help in managing moisture (see “Monitoring Moisture in Composting Systems,” October, 2000), most of the operators that we surveyed rely on visual inspection to determine when to add water (they may also feel and squeeze the compost to confirm their suspicions).

In climates with a dry summer season, including most of western North America, moisture loss is fairly consistent. Therefore, operators simply add water on a routine basis. For example, the facility in Carlsbad irrigates windrows for two hours each morning during the summer. An automatic timer controls the irrigation system. In other cases, facilities are unable to find the time and labor to keep up with the moisture loss. They simply add a fixed amount of water each week and settle for less than optimal moisture content. Although these facilities recognize when the moisture content is too high, they generally do not monitor materials to determine when more moisture is needed, because the materials are continually on the dry side.

Incorporating Moisture — How

Compost operators use a variety of methods to get the moisture into composting materials. They employ various combinations of tank trucks, hoses, windrow turners, sprinklers, and drip irrigation tubes. In-vessel or contained systems can add moisture by humidifying incoming air stream.

Using tank trucks to deliver water to windrows might be considered the conventional approach, and our small survey supports this. In windrow systems, tank trucks simply drive along the length of the windrow and discharge water onto or into it. There are several possible variations. Trucks may discharge by gravity or spray water on by pressure. Sometimes, a trough is created in the windrow so that water is retained. Implements also have been devised that inject water into the windrow. Typically, the windrow is turned soon after water is added. The turner also can follow behind the tank truck as it discharges water.

The Intervale and Earthworks composting operations use similar procedures for adding liquid feedstocks. The Intervale facility creates a trough in the windrow and then liquid is pumped into the trough through a 12-foot long pipe from the tank truck. (In most cases, the tank trucks delivering liquid feedstocks empty them directly into windrows.) After feedstocks are added, the windrow is turned. The procedure used for adding liquid manure at the Earthworks sites involves both a turner and tank truck. The turner first cuts a “V” in the top of the windrow. Then a tank truck empties manure into the windrow at the uphill end of the windrow. The liquid flows down the length of the windrow within the “V.” The turner agitates the windrow, starting at the downhill end.

Some of the newer windrow turners are equipped with water spray nozzles and appropriate plumbing for adding water during turning. Existing turners have been modified to do the same. Adding water during turning also provides a means to disperse innoculants and other dilute liquid feedstocks. Water can be delivered to the turner plumbing system via water tanks pulled by the turner or hose reels. Tanks provide more flexibility in site location and layout. However, unless well-planned, turning can be interrupted by the need to refill empty water tanks. Connecting the turner to a hose reel eliminates this problem but limits turner travel to the length of the hose. Among our survey participants, LRI and the Earthworks facility both use hose reels. In LRI’s case, the hose supplies water to a slotted pipe located above the side discharge conveyor of a Scat turner. Earthworks uses nozzles on the turner when adding fresh water. The water is delivered to the turner via the hose reel.

Irrigation equipment also is used to add water to composting systems, especially piles that are not agitated. The Carlsbad facility uses a fixed sprinkler system that applies treated wastewater to the surface of windrows daily. Three sprinklers are mounted on a retaining wall surrounding the composting pad. The sprinklers are spaced to distribute water evenly to the windrow. Drip tubes and large sprinkler guns also have been used. Sprinklers or nozzles are also used with agitated bed systems with nozzles positioned at one or more key locations over the bed. The water delivery system can be raised to allow the bed turning machine to pass.

If the need for additional water is not frequent, moisture can be incorporated by remixing and rebuilding piles and windrows. This occurs most commonly when wet feedstocks are added occasionally, rather than regularly. One approach is to form existing piles into a U-shaped bed and dump wet material in the center. Piles act as a reservoir. Wet feedstock is gradually absorbed by the dry materials surrounding it and then turned and formed into new piles or windrows. Texas Organic Products uses this approach to incorporate selected wet feedstocks.

In aerated systems, a more limited method of adding water is to humidify the inlet air. It has limited effectiveness and can only be used with positive pressure aeration. Incoming air at ambient temperatures, even when saturated, will not release moisture to a warm pile. The moisture that is added to the air stream simply prevents an equal amount from being evaporated. If a pile is low in moisture, another means of adding water may still be necessary. For negative aeration systems, it is possible to recycle the condensate from the air stream, as LRI does.

Summary

As mentioned earlier, it can be costly in terms of equipment, labor and water supplies to maintain adequate moisture through the composting process. Facilities should minimize the cost of obtaining water by using recycled moisture sources and wet feedstocks. Our survey suggests that most operators follow this advice. In general, the composting process goes faster if the moisture content remains within the optimum range. Therefore, the tradeoff is between the speed of composting versus the cost of getting moisture into the pile.

Many composters rely only on nature for moisture. In regions that receive regular rainfall, that is a workable option. However, when the rains don’t come, or don’t come on time, the process slows. Therefore, moisture conserving practices, like windrow covers and large piles, can make a difference (see “Conserving Compost Moisture,” October, 2000). There are other options that can help including shaping piles to trap precipitation and running windrows across the slope of the composting pad to catch runoff. In general, capturing and reusing site runoff is a sensible practice.

Acknowledgement: The editors of BioCycle appreciate the help of the following operators who contributed considerable information and their expertise to this article: Jim Doersam, General Manager, Texas Organic Products; Jeff Gage, Recycling Services Coordinator, Land Recovery, Inc.; David Hardy, Manager, California Bio-Mass, Inc.; Adam Sherman, Manager, The Intervale Compost Program; Robert Walker, Owner/Operator, Earthworks; John Waters, Deputy Director of Public Works, Carlsbad wastewater Treatment and Compost Production Facility.

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