15 - Downspouts in Sewer Wet Wells Protecting our community's health and the environment by providing solid waste and sewer collection services. www.cmsdca.gov Costa Mesa Sanitary District ….an Independent Special District Memorandum T o: Operations Committee From: Scott Carroll, General Manager Date: December 11, 2012 Subject: Downspouts in Sewer Wet Wells Summary Hydr ogen sulfide gases (H2S) are generated within the anaerobic slime layer formed on the submerged pipe walls. Hydrogen sulfide gases released from the slime layer rise into the airway portion of the sewer pipe and react with the bacteria and moisture on the pipe walls to form sulfuric acid (H 2 SO 4 ). It is the sulfuric acid that corrodes ferrous metals and concrete. H2S is the cause of the rotten egg smell in sewers and pumping stations. High levels of H2S are extremely dangerous to sewer workers and can cause death if protective measures are not taken. See Attachment A for stories regarding fatal sewer accidents caused by H2S. Staff has been proactively searching for a proven method to reducing H2S levels that is cost effective. Staff believes manufacturing and installing a downspout in sewer wet wells is that proven method. Staff Recommendation That the Operations Committee recommends to the Board of Directors the approval of staff’s plan to reducing H2S levels Analysis Before staff enters wet wells of pump stations they take readings for H2S levels. The levels identified below describe the dangers sewer workers face. Readings with high levels requires staff to wear respirator masks and comply with the District’s Respirator Plan.
Operations Committee December 11 , 201 2 Page 2 of 4 LOW 0.02 ppm 0.10 ppm No odor Minimal perceptible odor; May cause irritation of eyes, nose and throat MOD 10 – 50 ppm Headache, dizziness, nausea and vomiting, coughing and breathing difficulty HIGH 50 – 99 ppm 100 - 199 ppm 200 -300 ppm 500 – 700 ppm 700 – 1,00 0 ppm 1,000 – 2,000 ppm Respiratory tract irritation after one hour Coughing, eye irritation, loss of sense of smell after 2 – 15 minutes. Altered respiration, eye pain and drowsiness after 15 -30 minutes. Marked conjunctivitis and respiratory tract ir ritation after one hour of exposure Loss of consciousness and possibly death in 30 minutes to one hour of exposure Rapid unconsciousness, cessation of respiration and death Unconsciousness at once, with early cessation of respiration and death in a few minutes The Elden Pump Station, the District’s largest pump station that pumps 3,750 gallons of wastewater per minute, was averaging H2S levels of 70 -141 ppm with a maximum of 211 ppm. Staff placed this station as a high priority to reducing H2S levels to 1 ppm. Staff evaluated different methods to reduce H2S levels to reach a goal of 1 ppm, including chemicals, injecting gaseous pure oxygen, installing an oxidizing dry -air scrubber, cleaning the wet well bottom, and using a combined mixer and ozone in jector. Staff decided the most cost effective method to reducing H 2S levels was installing a
Operations Committee December 11 , 201 2 Page 3 of 4 downspout in the wet well. The down spout i s a creation of the District, and is a stainless steel custom fabricated fixture that is bolted to the wall where the in coming line enters the wet well. A rubber hose is attached to the fixture and the bottom of the hose extends down the wall to a point where the end of the hose is slightly su bmerged in the water. The down spout allows the incoming water to smoothly enter the wet well as opposed to cascading from above and causing turbulence. Decreasing the turbulence decreases the hydrogen sulfide that would normally off -gas into the atmosphere. This is a mechanical fix with a one -time small capital cost that permanent ly decreases H2S. This has proven to be very effective becau se after the down spout was installed, H2S levels at Elden Pump Station decreased significantly to averaging 7.6 ppm with a maximum of 46 ppm. The cost for manufacturing and installing a down spou t ranges from $6,500 -$8,000 depending on size and complexity . While the H2S levels have greatly improved, it is still not meeting the District’s goal of less than 1 ppm, so the District plans on installing a second down spout over the incoming gravity lin e. The first down spout handles the wastewater exiting into the wet well from the Irvine force main, which is a high volume force main. Because the down spout worked so well at the Elden Pump Station, the District decided to install a similar device in t he South Coast Plaza (SCP) pumping station. The SCP pumping station w as the ideal station for a down spout because shoppers would sometimes complain about the rotten egg smell when walking to and from their vehicles. For years, the District would spend th ousands of dollars a year applying chemicals in the wet well, which did help cut down on the odors, but H2S levels still did not reach the District’s goal of 1 ppm. After the down spout was installed and all th e chemicals in the wet well had b een disburse d, the District measured H2S levels in the wet well. The average H2S level was 1 ppm and the minimum was 0 ppm with the maximum at 15 ppm , whic h is outstanding! The new down spout will save the District nearly $20,000 a year on chemicals! Staff believe s the downspout is a very cost effective and proven method to reducing H2S levels. It is also noted that when the District designed the Irvine Pumping Station, a special bowl -manhole was used to have the wastewater swirl around the bowl and smoothly enter the wet well with a minimum of turbulence. This method also causes oxygen to re -enter the wastewater. Staff recommends that when wet wells are averaging 10 ppm or higher a downspout be installed. For stations that do not have a configuration conducive to a downspout, staff recommends utilizing other methods such as chemicals, injecting gaseous pure oxygen, installing an oxidizing dry -air scrubber, cleaning the wet well bottom, and Down spout in CMSD’s Elde n Pump Station wet well
Operations Committee December 11 , 201 2 Page 4 of 4 using a combined mixer and ozone injector. Strategic Plan Element & Goal This item complies with Strategic Element 1.0, Sewer Infrastructure and Strategic Goal No. 1.7, Pump Station Maintenance & Standardization Program. Legal Review Not applicable Financial Review The cost to manufacture and install a downspout is ap proximately $6,500 -$8,000, or an average of $7,250. Further analysis would have to be conducted on the remaining pump stations, but staff estimates 11 pump stations could possibly receive a downspout at a total cost of $80,000. None of the stations pla nned for abandonment are included in this analysis to receive a downspout; however, if public complaints are received about excessive odors then staff will consider installing a downspout. Public Notice Process Copies of this report are on file and will be included with the entire agenda packet for the December 11 , 2012 Operations Committee meeting at District Headquarters and on the District’s website. Alternative Actions 1. Do not approve staff’s recommendation and refer the matter back to staff for more information.