Effluent is the liquid waste stream from a brewery. Breweries use a significant quantity of water to make beer. Until the 1970s, it was common for breweries to use 30 to 37 l (8 to 10 gal) of water for every gallon of beer sold. This dropped to an average of between 18 to 22 l (5 and 6 gal) by 2010, and some breweries in water-scarce areas have conserved to 11 l (3 gal). The water not leaving the brewery as beer is the effluent.

Much is added to the waste stream. From the brewhouse, bits of grain and hops are most obvious, but the waste stream also includes kettle trub and the dilute wort that is too weak in sugars to use in brewing. From the fermentation and aging cellars come yeast and beer washings from tank cleaning. All stages in brewing use an assortment of cleaners, most commonly caustic soda (sodium hydroxide), acids such as phosphoric and nitric, and detergents and sanitizers, often containing chlorinated chemicals. Effluent is most critically analyzed for biological oxygen demand and total suspended solids.

Thankfully, water quality and sanitation have improved since the days of indiscriminate waste dumping. All breweries in the United States today are regulated as to where this waste effluent goes, and most pay fees both on the volume of wastewater and on the quantity of nonwater material it contains. Left uncontrolled, these costs can be prohibitively high.

As concerns increase about downstream water contamination and as municipal wastewater treatment plants become overwhelmed, breweries are increasingly pretreating this effluent before discharging it to drains.