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.
1. The Upward Flow Anaerobic Sludge Blanket as Used in 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 the bottom, the water requiring treatment is introduced at the bottom. From there it 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 gently in the water at the same speed as the water rises, stays perpetually suspended. In effect it is balanced in a horizontal layer, above the bottom, within the UASB reactor vessel while the micro-organisms digest it.
The extent to which the blanket is truly anaerobic can be questionable. The process was originally intended just as a high quality water treatment method. It is not known to the author how reliably this process can be used to produce a good biogas yield.
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. Individual particles can stay as long as needed, until they are eaten by the bacteria. For this reason the author assumes that there will be very little excess sludge created in UASBs. This reduces the problem of sludge disposal, and certainly reduces the cost of disposal of excess sludge remaining at the end of digestion. Sludge disposal can be expensive in other anaerobic digestion processes, especially where the material contains pollutants.
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 the heavy metals and other substance it may contain causing the problems which can occur when it is spread on land. This is done by a process known as “pyrolysis and/ or gasification”.
Why More Utility Companies Should Consider Installing Anaerobic Digestion Wastewater Treatment Facilities at Sewage Works
It makes economic sense, right across the US (and globally) for more water companies to invest in anaerobic digestion of sewage sludge. The EPA Says, and we quote: “Although the initial costs may be large, the digestion of food waste can be quite lucrative and the payback period can be less than three years, depending on the existing infrastructure at the wastewater plant”. See US EPA's Why-Anaerobic-Digestion.pdf
There are also, other benefits other than economic. In fact, wastewater treatment facilities are an ideal place to increase the diversion of food waste, when there is an anaerobic digestion facility available.
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The following is an excerpt from the above pdf, which outlines why more Utility companies should consider installing Anaerobic Digestion Wastewater Treatment Facilities at sewage works:
Existing Infrastructure – Many wastewater treatment facilities in the Pacific Southwest (US Region 9) and across the country, use anaerobic digesters to reduce the volume of the biosolids (sewage sludge) before they are taken off site. The anaerobic digesters produce biogas which is either flared or used onsite as an energy source. Therefore, the energy capturing infrastructure is already in place at many facilities.
Existing Expertise – Wastewater treatment facilities already have the on-site expertise and years of experience dealing with anaerobic digesters; vessels that are difficult to operate without thorough knowledge.
Located in Urban Areas – Wastewater treatment facilities are often located in dense, urban areas, where compost facilities are not. It makes logical sense for a highly populated area to ship organic waste to a nearby anaerobic digester where the energy content is recovered and the volume reduced. The residual can then be trucked to compost facilities, which are typically located farther from urban areas. via USEPA.
While many local governments and municipalities may be interested in processing food waste in anaerobic digesters at treatment facilities, they may feel that the cost is a limiting factor.
However, there are many things to remember before immediately discounting this technology based on cost.
Payback period: Although the initial costs may be large, the digestion of food waste can be quite lucrative and the payback period can be less than three years depending on the existing
infrastructure at the wastewater plant.
When a facility accepts food waste at a plant, they can charge the waste hauler a tipping fee for accepting the material. In addition, there is a significant amount of money that will be saved in
energy avoidance due to methane production. The excess energy can be sold back to the grid for profit. via USEPA.