Dear Carios dela, Odour ...

 Dear Carios dela,

Odour pollution is a threat not just too human health and welfare, but also to air quality, as it contributes to photochemical smog formation and particulate secondary contaminant emissions. Odour from wastewater treatment plants is caused by emissions, such as volatile organic compounds and sulphur compounds like H2S, Mercaptants.

SO4 2-   + 2C  + 2 H2O →2HCO3-  +  H2S

 It has traditionally been a low priority, but its importance has been increasing with more stringent environmental legislation.

There are two main types of technologies to treat odour

1.     Physical/chemical

2.     Biological

Physical/chemical technologies remove bad-smelling emissions by changing them with chemicals (chemical scrubbers), burning (incinerators) and by adsorbing the emissions with carbon

Biological technologies remove emissions using filter systems, made partially or completely from organic materials (Biofilters), that pass the emissions upwards whilst simultaneously trickling a cleansing solution downwards (bio trickling filters). Other biological technologies aerate the sludge or sewage with fine bubbles (activated sludge diffusion). 

Biofiltration and active carbon filtration require the most materials

Incineration requires the most energy.

“On the basis of their high efficiency at reducing odour and low environmental impact, activated sludge diffusion and bio trickling filtration appear to be the most promising technologies. The high water consumption of bio trickling filters could be reduced by using effluent or recycled water.”

Biological aeration tanks and secondary settling tanks must be aerobic to function efficiently. Keeping these units aerobic is the most important odour control consideration. The two major sources of objectionable odours in activated sludge tanks are development of anoxic or anaerobic conditions in the aeration tank and entry of dissolved odorous compounds into the aeration tank. As the first odour source, poor mixing or inadequate dissolved oxygen in the mixed liquor impairs the aerobic environment, causing settled deposits with resultant anaerobic conditions at the bottom of the tank. Because most diffusers can become clogged, routine cleaning is necessary. If the plant is equipped with swing-type, air-diffusion piping, operators can change clogged diffusers and flush air headers without emptying the aeration tanks. Mechanical aerators with the highest turbulence levels typically produce the highest odour and VOC emission rate, followed by coarse-bubble diffusers. Because of the low level of turbulence, fine-bubble diffusers typically have the lowest emission rate assuming all other factors.

As the second odour source, even with aerobic conditions, dissolved oxygen compounds may enter the tank with the primary clarifier effluent or with anaerobic return sludge flows; aeration then releases the compounds to the atmosphere. Aerosols formed during the aeration process can be carried by air currents far beyond the local tank environment. Both of these problems require control at the source.

Regards,

Prem Baboo