‘Flocculation’: How wastewater gets cleaned

Process of filtering what we flush is long, complicated

The San Antonio River Authority is looking for a supply of polymer — and interested vendors can request a sludge sample to make sure they’re the right fit.

So what does that all mean? And how does something called “flocculation” fit into all this?

We’re glad you asked. Prepare to learn everything you need to know — and maybe more than you want to know — about how the river authority treats wastewater.

While you might know the San Antonio River Authority — commonly known as SARA — for its work on the region’s rivers and creeks, it also provides sewer service for about 39,000 customers in the San Antonio metro area, and operates five treatment plants in eastern Bexar County.

When wastewater travels through the sewer system and arrives at the river authority’s treatment plants, it consists of about 99% liquids and 1% solids. The solids are any non-liquids you dump down the sink or flush down the toilet.

After arriving at the plant, the wastewater goes through a 32hour cleaning process, according to Leamon Anderson, the authority’s deputy director for utilities operations. The process includes straining things that shouldn’t have been flushed, like so-called “flushable wipes” and grease, which cause clogs, as well as using bacteria and microorganisms to break down waste.

At the end, the wastewater becomes two separate byproducts: clean water, which has been treated and disinfected with ultraviolet light, and that solid material, “which we call sludge,” Anderson said.

And that’s where the polymers and flocculation come in.

In simple terms, polymers are large molecules made up of chains of smaller molecules called monomers. A flocculent is a type of polymer used in treating wastewater. When added to the wastewater, it attracts the dissolved solids that are in the water, causing them to clump up and stick together in a process that is known as flocculation.

Anderson compared it to making a pitcher of lemonade from a powder: Once you put the mix into the water and stir it up, the powder dissolves. If you then added a flocculent, that mix would clump back together and separate from the water — which is exactly what needs to happen at that stage of the treatment process.

The polymer needs to be the right kind to match the wastewater at SARA’s plants, too. “Not every sludge is created equal,” Anderson said. Wastewater plants that have more industrial or commercial water might need different polymers than those that are primarily treating residential wastewater, like the river authority’s. To work most efficiently, the polymer needs to be a good match, Anderson said.

The sludge is also put through a belt filter press to try to extract as much water as possible, which leaves beyond a truly solid, dirtlike material. At that point, it’s known as “cake” — but you certainly don’t want to eat it.

The water removed at that point goes back through the treatment process, and the cake is taken to a composting facility, where it’s sold to customers who want compost for fertilizer.

SARA runs this process at two of its plants, Martinez II, near Interstate 10 and FM 1516, and Salitrillo, near Loop 1604 and Schaefer Road in Converse. Sludge from the other two plants is brought to those locations for treatment.

The polymer comes in 55-gallon barrels, and SARA uses 11 or 12 of those each month, Anderson said, at a cost of about $750 per barrel, or about $100,000 per year.

Salgu Wissmath/Staff photographer The clarifier is where sludge and clean water are separated at the S.A. River Authority’s Martinez II Wastewater Treatment Plant on Feb. 13. The plant services the East Side and Bexar County.