Anaerobic digestion is a process, commonly used in traditional wastewater treatments, in which microorganisms break down biodegradable material in the absence of oxygen. Co-digestion is a way of managing the process which, if properly exploited, can guarantee better results in biogas production.
Digestion is a biological process usually applied on the sludge produced in municipal wastewater treatment plants (WWTPs).
Specifically, this process is used for sludge stabilization, aimed at reducing the problems associated with the organic matter of the sludge
by exploiting the action of microorganisms that turn the organic matter into simpler and stable products.
Sludge stabilization can be performed aerobically or anaerobically. The advantage in adopting an anaerobic process, called anaerobic digestion, lies in the production of biogas (a mixture of gases mainly composed of methane, CH4 and carbon dioxide, CO2); biogas can be collected and used, as an energy source, to produce heat or electricity. The heat is normally used for heating the digesters themselves, which are the reactors where the anaerobic digestion process takes place.
The availability of excess volumes in the digesters of many WWTPs, and the low methane yield of sewage sludge (100-300 NL of CH4 per kg of volatile solid or organic matter fed, equivalent to about 0.3-9 Nm3 of CH4 per ton of sludge), make interesting the possibility to feed simultaneously the same digester with several different organic substrates, in order to increase the production of biogas: in this case, we call this "co-digestion".
Any organic substrate can be potentially used in co-digestion processes together with sewage sludge.
In urban areas, the OFMSW (Organic Fraction of Municipal Solid Waste) represents one of the most easily pursued options, often due to proximity between the WWTP and the solid waste treatment plant.
Moreover, OFMSW is one of the most productive substrates in terms of biogas (270-490 NL of CH4 per kg of volatile solid, equivalent to about 35-120 Nm3 of CH4 per ton of OFMSW).
In the area of Milan, 1.5 million tons of waste are produced annually; of these, approximately 225,000 tons are organic wastes.
At national level, every year about 14 million tons of OFMSW are wasted: of these, 25% is disposed off in landfills and 20% is incinerated; only 18% undergoes â€śvariousâ€ť biological treatments.
The co-digestion of OFMSW with sludge then may represent a "virtuous" choice for the treatment of such wastes, instead of using other techniques leading to higher environmental impacts (landfill or incineration).
Another co-substrate that is normally easily available in the municipal area, and that can be used in co-digestion with sewage sludge, is the expired food that is daily discarded from shops and supermarkets.
In Italy, more than 16.000 16â€™000 trucks carrying food expired or with preservation defects travel towards waste disposal facilities or landfills every year. 244â€™000 tons per year: a waste of food at different speeds, with bread at the top (15%), followed by vegetables (10%), milk (2.5 - 3%), yogurt (3 - 5%) and meat (1.6%).
The valorization of these products through co-digestion and biogas production, combined with a reduction at the source of the wastage of food, would certainly have a beneficial effect in reducing the environmental and economic impacts caused by such issues.
Water Service managers need to face two kind of issues when trying to adopt co-digestion processes in wastewater treatments; those issues are linked to the treatment of the materials both entering and leaving the plant.
A. Pre-treatment of incoming material
The quality improvement of the substrates fed in co-digestion has positive effects on the entire process, including: less wear and abrasion of materials and mechanical moving parts; a better quality of the compost produced from the digestate; lower amounts of deposits at the bottom of the digester, then reducing the risk of clogging of the digester.
B. Management of process waste
in most cases after digestion, the sludge produced (called digestate) undergoes a dehydration treatment which allows the water (supernatant) to be separated from the solid fraction. The supernatant is usually sent back to the top of the waste water treatment plant, while the solid fraction can be reused in agriculture in accordance with the local requirements.
Major benefits related to the co-digestion process can be summarized as follows:
higher biogas yield due to an improved balancing between nutrients and/or the development of wider and more diversified microbial communities
dilution of toxic compounds potentially responsible for inhibition during the process
moisture content optimization, allowing for simpler digester mixing equipment
increased process rate enabling smaller digester to be used
Benefits in terms of local management of the treatment and disposal chain of wastewater and municipal waste are listed below:
optimal waste management as a renewable energy source
local development of a synergistic action for an integrated management and processing system of wastes, eventually allowing for transport costs reduction
improved stabilization and quality of the final waste (e.g.: lower concentrations of heavy metals), a digestate of higher quality could be reused as fertilizer in agriculture.
The experimental activities included in PerFORM WATER 2030 regarding co-digestion processese aim at increasing biogas production. A pilot scale digester is foreseen as a means to adequately assess the benefits that can be obtained by co-digestion in comparison to the most commonly used techniques
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