Designing Leachate Collection for Compost Facilities

This is the first of a series of three posts that discusses leachate collection for compost facilities. There were three factors that prompted this blog.
1. A recent request from students who were designing a compost facility for a community in Northern British Columbia
2. Hurricane Sandy has been providing substantial precipitation in eastern North America, which significantly affects compost facilities and leachate collection facilities.
3. Last week I dropped off greenwaste at our community’s compost facility and observed that the leachate collection was already full, and the rainy season had hardly started!

What criteria do we use to design leachate collection? Do we design for maximum rainfall events? How do we take moisture loss during composting into account? How does temperature and precipitation interact when considering precipitation and evaporation potential?

Historically, yardwaste composting was done outdoors. Precipitation entered the compost and drained away into the soil, or into drainage ditches. Even in our high rainfall areas in British Columbia, we still have compost facilities conveniently located near a water discharge into a waterway or a creek.

Most compost regulations require collection and treatment of leachate. Leachate is defined as water from runoff or precipitation that has come in contact with organic material in a compost facility. The Organic Material Recycling Regulation (OMRR), administered by the BC Ministry of the Environment requires that all receiving, processing curing and storage occurs on an impervious pad, where any leachate produced must be treated before discharge into the environment.

Leachate must be collected and treated because the nutrients, BOD and fecal bacteria may be high. A study of four outdoor compost facilities in Western Washington found that leachate from these facilities contained 32-1600 mg/L of ammonium, 20-3,200 mg/L of BOD, and 200-24,000,000 MPN/100 mL of fecal bacteria (E&A Consultants 1997 http://www.cwc.org/wood/wd974rpt.pdf). Ammonium at very low concentrations is toxic to fish (http://www.thekrib.com/Chemistry/ammonia-toxicity.html), and fecal bacteria are an indicator of potentially pathogenic bacteria present in the leachate.

We have established that leachate needs to be collected. We now have to decide how much needs to be collected. The Compost Facility Requirements Guideline (http://www.env.gov.bc.ca/epd/mun-waste/regs/omrr/pdf/compost.pdf) suggests that the energy contained in the composting process is enough to evaporate a significant amount of moisture. This document cites a theoretical extended aerated static pile composting operation that produces enough energy to evaporate 25 mm of rain in 24 hours on the active composting area, and hence, produces no leachate. I will review this theory in a later blog.

My own experience is much different. The rate of evaporation depends on the temperature above the composting material, the humidity in the air, and the air movement. We started composting in windrows under cover in southcoastal BC in 1992, and found that the compost did not dry much during the winter months. In 1998, we observed that the compost in an covered agitated bed facility in south coastal BC did not dry significantly during the winter months. In 2005, in an agitated bed facility in upstate New York, we observed that the moisture content of the compost decreased from 65% to 60% in three weeks during the winter, and from 65% to 25% in 10 days during the summer. In Idaho, we observed that dairy farmers could add significant amounts of liquid dairy manure to turned windrows during the summer, but this ended in October. The primary reason is that it is the temperature above the composting material that dictates how much water is lost during the process. I will discuss this in more detail in a following blog.

During the winter months when evaporation rates are low and precipitation rates are high, we have to design leachate collection facilities that contain all precipitation, as we cannot assume significant evaporation. Incoming organic material during this period also is more likely to be saturated and will be unable to absorb additional precipitation.

In southcoastal BC, we can safely assume that we need to design a leachate collection facility to store precipitation from the the wettest six months – October through March (or November through April). Average precipitation information can be found at http://climate.weatheroffice.gc.ca/climate_normals/index_e.html. Let’s look at our own community of Abbotsford as an example. We have an average of 1029 mm of rainfall from October through March. Assuming an outdoor compost area of 1 hectare (100 m wide by 100 m long), we need to provide a leachate collection and storage of 10,290 cubic meters or 2.3 million gallons. In addition to that, we need to design for the highest rainfall in 25 years, which means that we should allow for an additional 10-20% capacity.

A leachate collection pond for a one hectare compost pad, assuming 10% additional storage, and assuming a 3 m depth, would measure 50 m wide by 75 m long. One factor that is not always considered is that the same rainfall that falls on the compost pad also falls on the leachate storage pond. This means that our 3 m deep storage pond needs to increase in size to 50 m wide by 115 m long!

There are two potential solutions to reduce the size of the leachate collection area. One is to cover the leachate collection pond so that precipitation does not contribute to the volume. The second is to create a deeper collection pond so that it has less surface area.

Covering the leachate collection pond is consistent with current recommendations for outdoor manure storages. However, it would be much more cost effective to cover the compost area or cover the composting material to eliminate the need for the collection pond altogether! We have to remember that storage of the leachate is only one part of the process. It still needs to be treated and discharged as well!

Creating a deeper leachate collection pond is another strategy to reduce the footprint. This is not possible in all areas. Some of our compost facilities are located in floodplains, where the groundwater level reaches within one meter of the surface. Leachate collection would need to be aboveground, as hydraulic pressure would cause challenges with any storage that extends below the water table. An above ground leachate storage pond then requires berms and pumps, increasing the cost.

In our cooler, high precipitation winters, where evaporation rates are low, we have to construct leachate collection areas that can contain the precipitation. In the next blog, I will consider the energy in the composting material and how it may contribute to winter evaporation, as suggested in the Compost Facility Requirements Guideline (http://www.env.gov.bc.ca/epd/mun-waste/regs/omrr/pdf/compost.pdf).

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