Sewer Surcharge Program

The purpose of the San Marcos Sewer Surcharge Program is to reduce excess amounts of oxygen demanding substances in our wastewater system. Grease, oils, and food waste are just some of the substances that clog up sewer lines and place a heavy load on the wastewater treatment plant. The program is also aimed at preventing discharges of mineral oils and toxic chemicals to the wastewater system. The Sewer Surcharge Program is intended to reduce such pollutants at the source to meet the goal of achieving cleaner water for San Marcos and Texas.

Determination of Charges

The City conducts wastewater sampling of businesses in the surcharge program once per year. The sample will be a composite consisting of at least 4 grab samples taken at approximately equal intervals during operating hours. Sampling is not announced in advance, but customers are notified when the city arrives to begin sampling.

When the sampling is complete, the customer is offered the opportunity to take a split sample which can be delivered to a laboratory of the customer’s choice for analysis. The results, along with chain-of-custody and laboratory quality data, must be submitted to the City within 10 days of sampling. If the results of the 2 laboratories are within 20%, the lower of the 2 readings will be used for surcharge calculation. If the difference is greater than 20%, then a portion of the original sample will be submitted by the City to a third laboratory and the results obtained will be used for surcharge calculation.

Customers whose COD (Chemical Oxygen Demand) is 350 milligrams per liter (mg/l)  or less will not be charged a surcharge for that year. Customers whose COD exceeds 350 mg/l will be subject to having the surcharge added to their monthly sewer bill. A testing fee of $24 is also charged.

Surcharged Rates

COD (mg/l)Rate ($/Pound)
351 to 500$0.109
501 to 600$0.180
Over 600$0.366
Surcharged Rates

Sewer Surcharge Formula

Surcharge = Volume of monthly water use (in millions of gallons) x 8.34 (weight in pounds of a gallon of water) x (COD-350) [350 is normal domestic wastewater] x surcharge rate


A business’ total COD = 700 mg/l Water use = 46,841 gallons/month or ________ = 0.046841 if expressed in millions of gallons


(0.046841)(8.34)(700-350)(0.345) = 47.17
 Plus monthly testing fee _________ + 24.00
 Total surcharge fee __________ = $71.17