Project Description

This project will consist of a biogas plant, compost facility, and a waste water treatment plant. The arrangement of all the facilities can be seen on the Site Layout, Attachment A.

The Biogas Plant is designed to process approximately 2,320 metric tons per day of manure from 60,000 dairy cows. This amount is based on the assumption that roughly 55% of the fresh manure stream is collected for anaerobic digestion. The other 45% will be utilized in the compost operation on site. The most efficient way to digest this large waste stream is to design a modular system, whereby all the material can be processed simultaneously yet allow for separate operation and maintenance functions. This Biogas Plant is designed as a facility with four identical units, each quadrant operating individually. Below is a general description of each unit:

The manure will be collected and either pumped through a piping system or hauled to the mixing tank where the manure is stored until it is pumped through a heat exchanger to elevate the temperature to the mesophilic temperature range of 92 °F to 104 °F. This heated manure is then pumped into the Continuous Stir Tank Reactor (CSTR) digester tank. Once inside the digester tank, the material is continuously mixed by a central, vertical agitator. It is here, in the digester tank, that the majority of the manure is degraded and the volatile solids converted into biogas with a methane concentration of approximately 60%. The degraded manure is then transferred to the post digester tank. The biogas generated in the digester tank is piped to the post digester tank headspace, which is a double membrane system that acts as the gasholder. The biogas stored in the gasholder is then fed into the inline hydrogen sulfide (H2S) scrubber, which reduces the H2S to below 200 ppm. This scrubbed biogas is then delivered to the gas preparation facility to be cleaned and pressurized for transmission to the utilities natural gas pipeline. The effluent from the post digester, or digestate, is pumped to a decanter centrifuge to separate the liquid and solids fractions. The solids fraction will be collected for the compost operation and a portion of the liquid will be pumped to the storage tank for liquid fertilizer, and the remaining liquid will be pumped to the waste water treatment plant.

The compost operation will consist of up to four fully enclosed buildings where windrows of raw organic materials can decompose to create a stable compost product. Solid digestate from the biogas plant along with raw manure will be mixed with other available organic materials to form the windrows. Once the compost is cured, the material will be moved to a finished compost transfer facility where it will be stored for eventual shipment to customers. Each compost building will be equipped with a scrubber system to purify the air, reduce odor and collect particulates.

A wastewater treatment plant will take all of the unused liquid digestate from the biogas plant and treat it for a broad range of uses including agricultural reuse. The treatment plant will include technology to reduce the high nitrogen and total dissolved solids concentrations to levels at or below that required to comply with the New Mexico Water Quality Control Commission standards. Technology to be employed is likely to include membrane and/or flash distillation of the high-strength stream before a more traditional treatment train. A partial diversion of some of the liquid stream will also be captured for storage and sale as a liquid fertilizer to capitalize on the favorable nitrogen content.