OPERATION OF ANAEROBIC DIGESTERS
The key to operating an anaerobic digester lies in balancing the acid formation and methane fermentation processes. This is accomplished by maintaining the desired temperature (95º F), ensuring mixing to promote contact between the organisms and the food as well as promoting even heating. Of high importance is FEEDING AND WITHDRAWING SLUDGE AT PROPER RATES. It is a good rule of thumb not to withdraw more than 5% of a digester content in a 24 hour period.
The type of sludge being digested is important. Raw sludge from a primary clarifier typically contains around 4 – 5% total solids, of which 70 – 90% is volatile matter. On the other hand, waste activated sludge can range from 0.2 – 6% total solids (high end due to thickening) with 65 – 75% being volatile. Raw sludge digests rapidly while waste activated sludge digest slower and less thoroughly, in part because of its lower volatile solids content at the beginning. Care must be taken to balance the feeding of raw sludge and secondary sludge in order to avoid upsetting the digester.
Feeding sludge to a digester is best done several times a day rather than all at once to avoid lowering the temperature of the digester too far at one time. Never change the digester temperature more than 1 degree per day. Pumping several times a day not only helps the digester but also help the clarifiers by avoiding holding sludge in primaries for too long and ensuring a consistently thick sludge. Every effort to pump as thick of sludge as possible should be made. This is because digester space is at a premium and sludge that contains too much water may dilute the buffering capacity of the digester. Buffering capacity refers to the digester’s ability to neutralize the organic acids that are being produced by the acid formers.
Buffering capacity depends on the amount of alkalinity contained in the sludge within the digester. If a digester does not contain enough buffering capacity, the pH of the sludge will drop. This harms the methane formers because they can only reproduce in a pH range of 6.6 to 7.6. If the pH falls too far, methane production will taper off and the digester is said to be “sour” or “stuck”.
Several tests are used to monitor the condition of the digestion process. These are;
• Temperature. A thermometer is usually located in the sludge recirculation line from the digester to the heat exchanger. A temperature of between 95º and 98º F should be maintained. Never change the temperature more than 1º F per day.
• Volatile Acid/ Alkalinity relationship (VA/Alk ratio). As long as volatile acids remain low and the alkalinity (buffering capacity) remains high, the digestion process will remain stable. Each treatment plant has its own acceptable ratio of volatile acids to alkalinity. However, this ratio is usually less than 0.1 part volatile acids to each 1 part alkalinity (10 times as much alkalinity as volatile acids). A CHANGE IN THE VA/ALK RATIO PROVIDES THE FIRST INDICATION THAT SOMETHING IS WRONG WITH THE DIGESTER. For this reason it is imperative that the VA/Alk ratio be monitored at least weekly.
• Digester Gas Content (CO2 and methane). The percentage of CO2 in the digester gas is an indication of digester performance, however, the VA/Alk ratio will change before the CO2 content begins to increase. Good digester gas will have 30 – 35% CO2 and 65 – 70% methane. If the CO2 content exceeds 42%, the digester is considered to be in poor condition. If the content of CO2 is greater than 45%, the gas will not burn.
• pH. pH readings are normally taken on raw sludge, recirculated sludge and supernatant. Because the VA/Alk ratio will show changes long before the pH actually changes, pH measurements should be used for recording purposes but not for process control. A pH of 7.0 – 7.6 indicates good operations.
• Solids Content. The total solids should be determined for the feed sludge(s), the recirculating sludge, withdrawn sludge and supernatant. A TS content of 3 – 6% is typical for sludge in the digester. Volatile solids reduction of the sludge is a key indicator of digester performance. Volatile solids reductions of 50 – 60% are not uncommon.
Key Indication: VA/Alk > 0.1:1, pH 7.0 – 7.2
Possible Cause: Excessive feeding of raw sludge, sludge withdrawal rate too high, shock-load
Possible Solution: Recirculate secondary digester sludge to primary digester to increase alkalinity and seed desirable organisms
Key Indication: VA/Alk >0.5:1, pH <7.0 and dropping
Possible Cause: Not correcting a “souring” digester
Possible Solution: Add alkalinity in the form of lime, soda ash or sodium bicarbonate to neutralize the excess organic acids
Key Indication: Large amounts of foam
Possible Cause: Start-up period, breaking of scum blanket, radical temperature change or over feeding
Possible Solution: Increase mixing, stop or decrease feeding, stabilize temperature
Low Methane Production –
Key Indication: CO2 >40%
Possible Cause: Start-up period or uneven control of temperature
Possible Solution: Stabilize temperature, control feeding rate and withdrawal rate
Large Amount of Solids in Supernatant
Key Indication: TSS > 1000 mg/L
Possible Cause: Drawing from wrong level (NOTE: supernatant should be amber in color)
Possible Solution: Draw from correct level (NOTE: supernatant is often very high in solids even after settling for several days)