by Ken Glotzbach, Wastewater Utility Manager, City of Roseville
The City of Roseville is actively researching and testing new technologies to reduce the odors caused by hauling sludge near neighborhoods close to the Pleasant Grove Wastewater Treatment Plant (PGWWTP). This article discusses the odor and what the city is doing to reduce that odor impact.
The PGWWTP, like almost all municipal wastewater treatment plants, uses biological treatment as part of the process to remove human waste from water so that the water is safe to release back into the environment. The key to this is using beneficial bacteria that feed on dissolved organic material and ammonia in the water that can be captured and separated from the water. These captured bacteria are "the sludge" being hauled from the PGWWTP on a routine basis.
Normally, these bacteria work in an environment that contains a controlled amount of oxygen which they utilize much like humans do as they breathe. Comparing this to a person's breathing, when the bacteria have oxygen available, much of what they exhale as they feed on the waste matter is odorless carbon dioxide. However, unlike humans, these same bacteria can survive in an environment that contains no oxygen which is the situation that is present once the bacteria are separated from the water being treated.
In the oxygen free environment, these bacteria use different mechanisms to "breathe" and what they produce are a class of chemicals called organic acids. Unfortunately, these organic acids have an odor that is offensive. This is why the sludge being hauled from the PGWWTP smells bad.
When the PGWWTP was originally planned and constructed, the area around the plant had no homes and the sludge was hauled on rural roads where these odors caused no problems. However, as development has moved closer to the plant and new roads have been built, the original rural hauling routes were eliminated. The city has tried a number of changes to improve the sludge odor as it is hauled including oxygen addition just before sludge is hauled from the plant and adding odor masking agents. None of these alternatives have been effective.
The obvious solution, sealing the sludge truck, has proved to be a challenge. Tanker trucks are effectively sealed, but the physical characteristics of the sludge make loading and unloading a truck like this impossible. We have also reviewed several covered bins but none have demonstrated the durability needed to do an adequate job of maintaining the seal.
Recently, the city has located a bin that seems to have an effective seal and will provide the needed durability. The city purchased one of these bins and expects to use it for sludge hauling within a few weeks.
The permanent fix for the odors is to build a treatment process at the PGWWTP called anaerobic digestion which stabilizes the bacteria population and eliminates their respiration and organic acid production altogether. The remaining solids from this process must still be hauled offsite for disposal but they do not produce the odors that are currently offensive.
Building of this process is planned as part of the PGWWTP's master plan to accommodate future growth. Based on the current growth projections, we expect to begin designing these improvements in about 2017 with construction to begin in the following year. These anaerobic digesters also produce methane gas which can be converted to electricity or be used to fuel vehicles. We are exploring these energy production opportunities to see if they might allow the city to begin construction of the digesters even sooner.