ELIMINATING ODORS IN WWTP

Problem

With new residential areas built near to older industrial sites which used to be located on the outskirts of cities, citizens are increasingly being confronted with the malodors of manufacturing companies. Furthermore, whereas in the past the workers of the company were the only ones living in the neighborhood of the factory, nowadays with increasing mobility, the company's neighbor may not be his best friend. Moreover people have learned to stand up, confront and even sue local government and business if they believe health and comfort issues are at stake. It is therefore that odour control is becoming an important factor for every environmental officer and production manager.

Odor is a very complex matter to quantify and qualify. It varies from person to person what is considered smelly. A malodorous substance called mercaptanes can be very pungent at extremely low concentrations of for example 5 ppb (parts per billion). Other bad odours such as H2S loose their smell at very high concentrations.

Facultative lagoons are typically associated with odors. The odors are primarily caused by two factors - mats of dead algae decomposing at the surface and along the sides of the lagoon, and hydrogen sulfide H2S produced by anaerobic conditions.

Decaying algal mats are caused by the periodic occurrence of excessive quantities of filamentous, blue-green algae. This group of algae, which flourishes in facultative lagoons during the summer months, does not settle like green algae but floats and accumulates at the surface where it decays in the sun, giving off noxious odors.

Hydrogen Sulfide is a colorless, very flammable gas. In low concentrations it smells like “rotten eggs”. However the sense of the smell is lost after 2-15 minutes of exposure making it impossible to smell dangerous concentrations. It is heavier than air and it is considered to be a very toxic gas. When H2S burns it produces another very toxic gas – Sulfur Dioxide (SO2).

Hydrogen sulfide is formed from sulfates in the wastewater that are produced in the anaerobic environment at the bottom of the lagoon, as well as from the decomposition of materials in the bottom sludge layer.

As water approaches its freezing point and cools below 4°C, its density begins to decrease until it freezes at 0°C. During winter, stratification occurs as the warmer, denser water is trapped beneath the colder surface layer. When the colder surface water warms in the spring to about the same temperature as the water beneath it, de-stratification occurs and the entire water column mixes together because of similar density (spring turnover). During this spring turnover, the odorous H₂S gas trapped in the anaerobic zone at the bottom of the lagoon is released to the atmosphere as the lower layer of water is exposed to the surface.

An inadequately aerated or organically overloaded aerated lagoon can also produce odors through the year. Both of these scenarios result from depleted oxygen levels and subsequent anaerobic conditions.

Solution

In order to obtain a complete aerobic process, an aeration system is necessary. When the system is operated within specified design parameters, no odors will be produced.

For a PNW lagoon, the air is introduced through ADS fine bubble diffusers at the bottom of the lagoon at the sludge-water interface. The Oxygen contained in the air is provided to the bacteria and microorganisms resulting in aerobic conversion of sludge to carbon dioxide, water, and inert ash, none of which smell.

Elimination of thermal stratification and prevention of spring turnover are assured by ADS aeration systems, which continuously mixe the water column.

Result

When the system is operated within specified design parameters, a PNW aerated lagoon system will produce odor free treatment conditions at all times.