Aerobic Digestion

There is not much involved in the operation of aerobic digesters. Sludge feed rates are based on the need to remove solids from the secondary treatment process. Sludge removal rates are determined by the level in the digester and by the availability of sludge dewatering and drying units.  Around 1.0 mg/L D.O. should be maintained or the digesting sludge can produce a “rotten melon” odor that becomes progressively more offensive.  Aerobic digesters should be taken off-line and completely cleaned at least once every three years in order to preserve the tank volume (remove sediment) and inspect submerged equipment.  Aeration in aerobic digesters are routinely stopped to allow for supernating the digester (supernatant is returned to the head of the plant or primaries for further treatment). Monitoring tests that should be done at least weekly include;

• pH. The pH of the digesting sludge should be determined and recorded. The pH of aerobic digester sludge should always be above 7.0. If it is not, a problem in the secondary treatment process is indicated. An example would be un-wanted nitrification causing acidic conditions in waste activated sludge. A source of alkalinity such as lime may have to be added to the digester in this situation to maintain a neutral pH.

• Total Solids. The TS of the feed sludge, digesting sludge and of sludge withdrawn from the digester should be known. TS in aerobic digesters typically range from 1.5 – 4%.

• Volatile Solids. The volatile solids content of the feed sludge, digesting sludge and sludge withdrawn from the digester should be known. The reduction of volatile solids through the digester gives a measure of the effectiveness of the digestion process. Over time, the volatile solids reduction will decrease. This is due to sediment that reduces the digester’s volume (resulting in a lower digestion detention time). If the volatile solids reduction falls to unacceptable levels, cleaning of the digester to restore detention time is indicated.

• Dissolved Oxygen. D.O. in the digester should be measured with a calibrated D.O. meter. In order to avoid serious odor production, at least 0.5 mg/L of D.O. should be maintained.

CHEMICAL SLUDGE STABILIZATION

Sludges which are not biologically digested can be made stable by the addition of large doses of lime. THE ADDITION OF LIME TO SLUDGE TO PREPARE IT FOR ULTIMATE DISPOSAL IS NOT A COMMON PRACTICE. Chemical stabilization is usually considered a temporary stabilization process and finds applications at overloaded plants or at plants experiencing digestion facilities upsets. The main drawbacks to chemical
stabilization are the cost associated with the large quantities of chemical required and the quality of the end product.

Lime stabilization is accomplished by adding sufficient quantities of lime to the sludge to raise the pH to 11.5 12.0. This extremely caustic condition kills virtually all organisms in the sludge, thus preventing biological changes (temporarily) and killing off pathogenic organisms such as bacteria, viruses and parasites.

The best way to determine the amount of lime required to raise the pH of a particular sludge to within the desired range is to perform a bench scale test using 1 – 2 liters of sludge and then calculate the volume needed for full scale based on the results. An important drawback to lime stabilization of sludge is that, unlike other stabilization processes, the overall mass of solids is not reduced. In fact, it increases.