What is the relationship between digester and biogas in the context of the input sentence. Choose an answer from: product/material produced; manufacturer; distributed by; industry; position held; original broadcaster; owned by; founded by; distribution format; headquarters location; stock exchange; currency; parent organization; chief executive officer; director/manager; owner of; operator; member of; employer; chairperson; platform; subsidiary; legal form; publisher; developer; brand; business division; location of formation; creator.

To increase the value of our high-grade and fuel ethanol and to reduce the cost of operation of our production plant, we are currently implementing several upgrade projects related to the California plant, including: Number one, building two new distillation columns and related systems to produce high purity U.S. Pharmacopoeia grade alcohol for sanitizers known as USP grade; Number two, installing five new segregated stainless steel tanks for USP high-grade alcohol storage and loadout, increasing our storage capacity by more than 250,000 gallons; Number three, completing the installation of the new $7 million zeolite membrane dehydration unit from Mitsubishi to reduce natural gas use at the alcohol plant by replacing our molecular service with electrically powered equipment, which will reduce the carbon intensity of our fuel ethanol and is partially funded by a $1.5 million energy efficiency grant; Number four, adding high efficiency heat exchangers to reduce natural gas use at the alcohol plant, reimbursed by a $1.3 million energy efficiency grant; Number five, installing a solar panel micro grid array with battery backup and an artificial intelligence energy management system to replace natural gas with solar electricity while optimizing energy use throughout the alcohol plant, primarily funded by an $8 million California Energy Commission grant; Number six, designing and building a mechanical vapor recompression or known as MVR system to significantly reduce petroleum natural gas use, partially funded by a $6 million California Energy Commission grant; Number seven, constructing a dairy biogas digester cluster and pipeline to deliver renewable natural gas to the alcohol plant from an initial two dairies this year with planned expansion to an additional 15 dairies next year, along with an interconnection to the utility gas pipeline.

Answer: product_or_material_produced