Taking steps now for better odor control


The Water Resource Recovery Facility (WRRF) receives millions of gallons of sewage a day from the community, then treats and transforms it into compostable solids, renewable energy and recycled water. Some of the recycled water is delivered back into the community for irrigation while the remainder is released to San Luis Obispo Creek to support local habitat for species like protected Steelhead. As vital as the WRRF’s role is, it can be odorous—a nuisance that’s exacerbated in warmer weather. To better support our neighbors, controlling odor is a key design consideration of the SLO Water Plus upgrade now underway.


Generally speaking, foul odors at treatment plants originate from the breakdown of organic compounds. A natural byproduct of this breakdown is hydrogen sulfide (H2S), which gives off a strong, rotten egg smell. Because H2S does not dissolve in wastewater, it’s released into the atmosphere, producing an offensive odor. The COVID pandemic has lowered water consumption citywide, which exacerbates these conditions as wastewater stays in the collection lines longer before reaching the treatment plant.


Odors are most often present at the collection, primary treatment and solids removal stages of the WRRF treatment process. Here’s a snapshot of how the modernized odor control resulting from the SLO Water Plus upgrade will better support our valued neighbors:



Collection Stage


Wastewater is “freshened” with oxygen. As wastewater flows from homes into the WRRF through a system of pipes, a lack of oxygen in the water can result in septic conditions that produce odor. To reduce that effect, the sewage is “freshened” by pumping air into the into the raw sewage. This step will continue following the site upgrade.


Expanded flow capacity means less sewage held in ponds. The population and industry of SLO have changed significantly since the WRRF was built more than 100 years ago. With growth, the WRRF has experienced a corresponding increase in the daily volume of wastewater it receives and processes. Currently, when the flow is higher than the capacity of the facility’s microorganisms to do their job, the excess sewage is diverted and temporarily stored in the Equalization (EQ) Pond located near Prado Road.


To help minimize odors escaping from the EQ Pond, staff currently limit how long the wastewater is retained, and it’s cleaned as frequently as possible. Still, the process can be odorous and high temperatures can reduce the effectiveness of this step, as witnessed by anyone driving down South Higuera Road on a hot day.


The SLO Water Plus upgrade will increase flow capacity to better accommodate current and future conditions, eliminating about 800 thousand gallons of partially-treated sewage from being stored next to Prado Road. The storage pond will be retained to support the WRRF only during extreme wet weather events.



Primary Treatment Stage


Addition of covers and foul air ducting at primary clarifiers. One of the first treatment processes wastewater encounters are two clarifiers which remove 60%-80% of the solids in the water. Wastewater enters the clarifier from a center column and exits by overflowing a weir or low dam that regulates flow, then into a launder or trough. The launder takes the water to the next stage of the treatment process where fine screens remove what solids are left, allowing only wastewater to move on to the next stage.


Because this step was identified as a source of odor, covers have been designed. In addition, ducting will route this foul air from the launders and the fine screens to a new biological odor control system described below.



Biosolids Removal Stage


Sealed containers replace open truck storage. The current WRRF design limits access for large semi-trailer trucks to a single section near Prado Road. These trucks haul more than 135,000 pounds of biosolids each week to Santa Maria to be converted into compost and fertilizer. To accommodate truck access limitations, staff use a screw press to discharge solids directly into the back of an open dump truck. Once the truck is filled, the biosolids are dumped next to Prado Road, and then stored until enough is compiled to fill a semi-truck, after which they are loaded and hauled from the site. This process is a recognized source of odor from the WRRF.


The SLO Water Plus upgrade will introduce two design improvements that will effectively eliminate this odor source. First, solids will come off the screw press into sealed transport containers called Level Lodors so the solids will never be exposed to open air. The second improvement is described below.



Fully automated solids handling unit eliminated drying beds. Currently, biosolids handling produces overflow from digesters that requires the use of open air drying beds, an odor source located next to the Bob Jones Trail. To avoid overflows of solids, valves are opened daily, allowing about 50 gallons of digested sludge to pour into the drying beds each time. The sludge can stay in these beds for up to five months, slowly drying. Once the sludge is 15%-16% solids, it’s removed and transported away with the other solids to produce compost and fertilizer.


The SLO Water Plus upgrade will introduce a fully automated, 24/7 solids processing process that takes up a significantly smaller footprint (reduced from 1 acre to 600 square feet). The upgrade will eliminate the daily need to dump biosolids in the drying beds. Two drying beds will be retained for emergencies which will open up space to improve access for semi-trucks for easier transport of solids offsite.



New odor control facility will be introduced. The SLO Water Plus upgrade will repurpose a former concrete tank, built in the 1920’s, into an odor control facility that will receive foul odor from several odor sources via a network of ducts. Odor will be captured, and transported from eight processes and will treat over 14 million cubic feet per day of foul air.



When SLO Water Plus wraps up construction in 2023, neighbors will be hard pressed to find the WRRF by nose alone. To stay tuned to these and other campus improvements, subscribe for SLO Water Plus project updates now.


© City of San Luis Obispo 2019