In my last two blogs, I discussed leachate collection requirements for compost facilities in our cooler, high precipitation climates in Southcoastal British Columbia. I recommended that a one hectare outdoor composting pad in Abbotsford receiving 1029 mm of precipitation between October and March needs to collect 1029 cubic meters of water (2.8 million gallons). I mentioned how this estimate is in stark contrast to the Compost Facility Requirements Guideline (http://www.env.gov.bc.ca/epd/mun-waste/regs/omrr/pdf/compost.pdf) that suggests that I may have very little to no leachate at all if I design my composting system correctly!
Let’s assume that this information in the guideline is correct. Let’s explore this a bit further as there may actually be a business opportunity if this is correct. Let’s use the same example from the guide that I used in the last post. Its a one hectare site with 3 m high by 7.6 m wide windrows with a1.5 m alley between the windrows. The guideline suggests that with a daily precipitation of 25mm, which produces 245,000 L of water, only 37,000 L would have to be collected because the remainder will be evaporated (under the right conditions of course!). This means that according to the guide, I can evaporate up to 208,000 L of liquid using this windrow system.
If that is true, I have one of two choices. Its a business decision. I can keep my 1 hectare site uncovered and compost without generating any leachate as long as the average rainfall is less than 21 mm per day. The other choice I have is to cover the compost site (put it under a roof) and sprinkle up to 21 mm per day of wastewater onto my composting windrows! Why would I do that? Tipping fees. The cost of processing wastewater is expensive, because it is costly to remove nutrients from water. Evaporation would then be a simple way to do this!
What initially would seem like a ridiculous idea to cover a one hectare composting pad may not be such a bad idea after all. Assuming a cost of $20 per square foot for a building, the total cost of the building would be just over $2 million. What is the revenue opportunity for receiving and treating wastewater? If we can evaporate 208,000 L per day, at a wastewater tip fee of $ 60 per tonne, our revenue would be $12,400 per day. In a year, that would generate over $4.5 million dollars! And I would save the environment because I am successfully separating nutrients from water, which is something that our wastewater plants are only partially successful at.
Furthermore, if the wastewater contained any BOD or available carbon, this too could be used as additional heat potential to evaporate water. This means that I may be able to process even more liquid than was predicted by the guideline!
Sadly enough, the evaporation rate as suggested in the guide simply is not true. Which means that there really is no business opportunity with evaporation of wastewater from an outdoor windrow process!
I mentioned in my last blog that the amount of evaporation would be dependent on the outdoor temperature because air at low temperature cannot hold much moisture compared to air at 60 C. So, what if I have a negatively aerated static pile system like the last example in the Compost Facility Requirements Guideline where the air is drawn downward, and therefore stays hot? Yes, a negative air system would successfully remove more moisture because the air leaving the composting pile is hot. There are a couple of factors to consider:
1. the temperature of this evaporated air will decrease, resulting in significant condensation (liquid), and
2. the condensation would then be mixed with leachate, so it is no longer clean. I had addressed this in a previous blog (http://www.transformcompostsystems.com/blog/2012/06/04/understanding-moisture-loss-during-composting/)
Biodrying or evaporative loss of moisture during composting has been done successfully with a couple of technologies. It requires positive aeration, and an insulated cover where the air can be removed while it is still 60 C and carries a lot of water.
In the meantime, I would encourage that the Compost Facility Requirements Guideline be updated to reflect a more accurate estimate of the potential leachate that can be collected.