There are two distinct uses of Anaerobic Digestion in Wastewater Treatment:
- As a treatment process in its own right for primary sewage treatment/ organic industrial effluent, as known as a “UASB”
- As a method of treating the sludge produced by Wastewater Treatment Plants.
We provide articles on both subjects, below:
1. The Upward Flow Anaerobic Sludge Blanket as Used in Wastewater Treatment
The Upward Flow Anaerobic Sludge Blanket or AUSB process is a specialist use of anaerobic digestion wastewater treatment. It is quite rare to see, and it is based upon the idea that the particles in any sludge are gradually falling within the liquid they are in. By creating a tank which is cone shaped, resting with the apex of the cone at he bottom, the water requiring treatment is introduced at the bottom and flows constantly upward, very slowly. The flow rate is then balanced with the geometry of the tank so that a “blanket” of sludge (which is particles of organic matter and bacteria constantly falling in the water) stays balanced within the UASB reactor vessel.
The extent to which the blanket is truly anaerobic would be questionable, and as the process was originally intended just as a high quality water treatment method, it is not known whether this process has been used to produce biogas. However, it is a clever way in which to create a process in which the particles which digest slowly would tend to reside in the reaction area/ sludge blanket as long as needed until they are eaten by the bacteria. For this reason the author assumes that there would be very little sludge created. This removes the problem of sludge disposal, and certainly reduces the cost of disposal of excess sludge remaining at the end of digestion, which can be expensive in other anaerobic digestion processes.
2. Treating the Sludge Produced by Wastewater Treatment Plants
This is the process of decomposing organic matter of municipal sewage sludge anaerobically under conditions of adequate operational control. During the digestion of sludge, it is broken up into three different forms:
(i) digested sludge which is a stable humus like solid matter with reduced moisture content
(ii) supernatant liquor which includes liquefied and finely divided solid matter, and
(iii) gases of decomposition like methane (CH4), carbon dioxide (CO2), nitrogen (N2) etc produced by microbial digestion.
The digested sludge is de-watered, dried up and used as sewage sludge fertilizer while the gases produced are used as fuel or for driving gas engines. The supernatant liquor is re-treated at the treatment plant along with the raw sewage. The tanks in which sludge digestion is carried out are called sludge digestion tanks.
The Process Of Digestion Of Sludge
Three stages are known to occur in the biological action involved in the process of digestion of sludge for anaerobic digestion wastewater treatment. These are (1) acidification (2) lysis/ liquefaction or a period of acid digestion and (3) gasification or conversion of acids into methane and carbon dioxide.
As the fresh sewage-sludge begins to decompose anaerobically, bacteria attacks easily available food substances such as carbohydrates (sugars, starches, and cellulose) and soluble nitrogenous compounds. The products of decomposition are acid carbonates, organic acids with gases as carbon dioxide and hydrogen sulfide. Intensive acid production lowers pH value to less than 6. Highly putrefactive odors are evolved.
In this stage, the organic acids and nitrogenous compounds of the first stage are liquefied i.e., transformed from large solid particles to either a soluble or finely dissolved form. The process is brought about by hydrolysis using extra cellular enzymes. It is during this period, that the intermediate products of fermentation in other words, acid carbonates and ammonia compounds, accumulate and the resulting gasification into H2 and CO2 is at a minimum. The pH value rises a little to about 6.8, odor is extremely offensive and the decomposing sludge entraps gases of decomposition, becomes foam and rises to the surface to form scum. This stage is known to last much longer than the proceeding stage of acidification and hence also termed as acid regression.
3. Sewage Sludge Biogas Production
It is the stage when more resistant materials like proteins and organic acids are broken up. Large volumes of methane gas of high calorific value, along with comparatively smaller volumes of carbon dioxide are evolved. The pH value goes to the alkaline range i.e., above 7 and a tarry odour appears. Gasification finally becomes very slow; the sludge becomes well adjusted and is stable enough for disposal. This stage in the digestion of sludge is also termed as alkaline fermentation.
The remaining sewage sludge after digestion can be disposed of sustainably, and without metals and other substance it may contain causing a problem when it is spread on land, by a process known as “pyrolysis and/ or gasification”.