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Vermifiltration: A low-cost, environmentally sustainable way to treat dairy waste?

Posted by Embrey Bronstad | February 15, 2021

This is part of a series of posts highlighting work by Washington State University researchers through the Applied Bioenergy Research Program of the Agricultural Research Center at Washington State University, College of Agricultural, Human, and Natural Resource Sciences.

What do you call the worm that ate Mozart? A DECOMPOSER!

But for one central Washington dairy, a treatment system is using worms, not to decompose manure but to address the knotty and chronic issue of excessive nutrient levels. And now WSU researchers are studying just how helpful those worms might be to the Washington dairy industry.

Vermifiltration is a wastewater treatment process where wastewater is introduced to a filter (often wood chips) that is also literally crawling with worms. The wastewater keeps the filter media wet and encourages the growth of a thin film of microorganisms which degrades the organic fraction (i.e., manure) of the wastewater. The biofilm also helps reduce nitrogen levels by converting a portion of the ammonia to harmless nitrogen gas, which simply releases to atmosphere. The worms aerate the filter, providing oxygen to help support the aerobic parts of the biofilm, but the worms also graze on the biofilm, keeping it in check.  Of course, as the worms eat, they also generate their own…fertilizer (ahem), which is very commonly used as a beneficial soil amendment. The worms themselves are also a potential source of revenue, making the system not only environmentally beneficial, but cost-effective.

Hand holding clump of worms and worm castings
The Decomposers (Photo Credit: Sina Pram, Biofiltro)

Vermifiltration has been used successfully in industrial and municipal wastewater treatment. Using this technology for dairy wastewater, however, is a new and innovative application. Using the one installation currently in place at a dairy in central Washington, Dr. Pius Ndegwa, a professor in the Department of Biological Systems Engineering, and his team are delving into various aspects of this novel utilization.

Their preliminary results indicate the technology is extremely promising. Total nitrogen in the dairy wastewater, a critical parameter that governs land application rates of manure, was reduced by up to 91% by vermifilter treatment.  The filtration process also reduced  chemical oxygen demand (COD; an indicator of the levels of undesirable organics mentioned previously) by moderate levels (up to ~50%) and reduced upwards of a third of the total solids; high concentrations of solids in irrigation water can cause plant roots to lose rather than retain water.  Slightly more than half of the total phosphorus (~57%) was removed as well, likely as a consequence of the total solids removal. This level of nutrient removal makes the utilization of wastewater as fertilizer more feasible for the farmer, as the wastewater (which is heavy to transport) can be applied at higher rates to nearby fields.

Future work by Dr. Ndegwa’s team will look at operational questions such as how well nutrients are removed based on how high their concentrations are in the wastewater. This parameter will be of particular interest as they also plan to look at not just raw manure but effluent from an anaerobic digester (AD). Anaerobic digester effluent has higher concentrations of ammonia than raw waste, which could impact the total nitrogen removal efficiency of the filter. But, if it works, AD coupled with the relatively low-cost and low-energy consumptive vermifilter could provide a full waste treatment process that reduces nutrients, generates energy, and provides two additional saleable products: worm castings and earthworms. The increased use of these biologically-mediated technologies could be a boon for dairy farmers as both an economic and environmentally beneficial solution for dairy waste treatment.

Two long, low beds with wood chips. Irrigation risers and j-vents span across each structure.
Vermifilter at Dusk: Irrigation lines and air vents (Photo Credit: Sina Pram, Biofiltro)

Stay tuned for the results of this pairing of technologies, as well as evaluations on nutrient removal as a function of worm density and wastewater application rates!

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