Co-digestion of Algae from Algal Turf Scrubbers in Farm-based Digesters to Increase Profitability and Reduce Nutrients to the Chesapeake Bay Watershed

View the project final report

Commodities

Practices

• Energy: anaerobic digestion

Maryland farmers can install algal turf scrubbers (ATS) to generate nutrient trading credits. In an ATS, pond or river water is pumped across a flow-way which allows algae to seed and grow in a contained “bloom” on land that traps nitrogen and phosphorus in the algal turf for easy harvest. An ATS allows for upstream removal of nutrients to improve water quality downstream, while generating nutrient credit payments for farmers. Installation and operational costs of an ATS may be partially offset by using the weekly algae harvest to create value-added products for the farm. This experiment will use anaerobic digestion (AD) to process ATS algae into methane (CH₄)-enriched biogas and fertilizer via co-digestion with traditional agricultural AD feedstocks (dairy manure, food waste, and poultry litter). The biogas can be used on-farm for electricity production to increase farm income. Fertilizer will be produced from AD effluent, which is commonly land applied on-farm. Thus, the nutrients trapped by the algae will be recycled for farmers to use again.

Two phases of experimentation will be performed. In Phase I, a batch-scale AD test will measure the volume and quality of CH₄ produced from co-digestion reactors over a 45-day period. In Phase II, the effluent from the batch reactors will be used to fertilize potted lettuce to assess the quality of this fertilizer compared with commercial fertilizer. These experiments will demonstrate how farmers utilizing AD can supplement their digesters via co-digestion with algae and improve the economic benefits provided by both ATS and AD.

Objective 1. Quantify the volume and quality of biogas produced from co-digestion of ATS algae with traditional agricultural AD feedstocks: dairy manure, poultry litter, and food waste.

Hypotheses:

-H_a1.1: Co-digestion of ATS algae will yield a higher volume of methane (CH₄) than produced from AD of algae or the agricultural feedstocks alone.

-H_a1.2: Co-digestion of ATS will yield biogas with a lower concentration of H₂S, and thus be of higher quality, than biogas produced from the agricultural feedstocks alone.

These hypotheses will be addressed using a batch-scale AD reactor experiment. ATS algae grown in Maryland will be digested alone and via co-digestion with dairy manure, poultry litter, and food waste over 45 days, or until biogas production ceases in all reactors. It is expected that biogas from reactors containing algae will experience elevated biogas production, and a corresponding increase in CH₄ production, due to the introduction of more complex substrates for AD bacteria to utilize. Additionally, it is expected that this biogas will have lower H₂S than reactors without algae due to the iron content of algae bonding with sulfur to produce iron sulfide precipitates instead.

Objective 2. Determine how co-digestion of ATS algae with traditional feedstocks affects the quality of fertilizer produced from the digester’s effluent.

Hypotheses:

-H_a2.1: Lettuce seeds fertilized using effluent from co-digestion of algae will produce biomass equal to or greater than seeds fertilized using commercial fertilizers or effluent from AD systems utilizing agricultural feedstocks alone.

-H_a2.2: Lettuce grown using effluent from co-digestion will not contain a higher concentration of sodium or heavy metals than lettuce grown using commercial fertilizers or effluent from AD systems utilizing agricultural feedstocks alone.

These hypotheses will be addressed using a lettuce growth experiment carried out after the reactor experiment described in Objective 1. Effluent from the reactors will be used to fertilize buttercrunch lettuce seeds to compare its effectiveness with commercial fertilizers. It is expected that co-digestion will dilute any salts or heavy metals present in the algae sufficiently to prevent them from interfering with lettuce growth or increasing their concentration in the lettuce biomass, and thus verify the utility of the co-digestion AD effluent for land application.