Hydrogen sulfide gas and gaseous ammonia are the main odor-causing by-product of anaerobic digestion. Odor can also be caused by volatile acids in the incompletely digested sludge. Hydrogen sulfide can cause corrosion problems when the gas is used in boilers or engine generators. Therefore, the gas should be chemically scrubbed to remove hydrogen sulfide before use.
A leak-free gas-handling system will not cause an odor problem. Odor problems in an anaerobic digestion system are associated primarily with supernatant liquid and with storage and dewatering of digested sludge. Supernatant is not drawn off from single-stage digesters. In two-stage systems also, prethickening of sludge can eliminate, or at least reduce, supernatant discharge volume.
Dewatering operations of digested sludge create odors that are very difficult to control, especially when open dewatering equipment such as a belt filter press is used for dewatering. An option is to scrub the ventilation air around the dewatering equipment and from the digested sludge day tank using chemicals. Odorous air can also be treated by passing the air through an odor control biofilter. Because of their enclosed design, centrifuges contain odorous air, thereby facilitating efficient off-gas collection for treatment. Belt filter presses are presently available with factory-built enclosures for the same reason.
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