The anaerobic digestion of biodegradable tannery wastes

The tanning industry produces large amounts of organic waste in the form of fleshings, trimmings, split off cuts and shavings from raw hides and skins as well as sludge as a result of wastewater treatment. While the waste from raw hides can be processed into usable products (often in rendering plants) the wastewater sludge is proving more problematic. Conventional handling of sludge involves landfill sites; however, this option is discouraged (in the EU it is not permitted any more) as it fails to resolve the solid waste disposal problem in an ecologically acceptable manner. The organic waste undergoes biodegradation and emits greenhouse gases, in particular methane (a gas with over 20x higher greenhouse effect compared with carbon dioxide) and to a lesser degree nitrous oxide (300x higher greenhouse effect). Furthermore, it poses potential odour as well as land contamination problems.
The disposal of waste is, therefore, a serious problem and the importance of technological measures to combat the environmental challenges is now being increasingly recognised. In light of current developments the adoption of environmentally friendly waste-to-energy recycling technologies has become an attractive option for the tanning industry.
For larger tanneries on-site anaerobic treatment of biodegradable tannery waste is an interesting option. Anaerobic Digestion (AD) is a biochemical process during which complex organic matter is decomposed by various types of microorganisms in the absence of oxygen. The process of AD is common to many natural environments such as the stomach of ruminants and is the reason for greenhouse gas emissions in landfill. But, by carrying out these processes under controlled conditions the organic matter is stabilised in the form of digested sludge and biogas is obtained, which is a mixture of methane (65%), water vapour (5%), hydrogen sulphide (up to 1%) and the remainder is carbon dioxide.
Although biogas is simply a by-product of AD it has become the major topic of attention because of its energy potential and renewable nature.

Substrates
Wastewater treatment and pre-tanning process steps are the major sources of biodegradable tannery waste. Wastewater sludge from primary treatment is often blended with that from secondary treatment before thickening to about 5% total solids. With conventional sludge handling, the thickened sludge is then dehydrated; however, for AD this is not required since the maximum solids content to run the process should be around 10%. The sludge contains about 60% of organic matter, while the remaining inorganic matter is mostly lime.
Although good use of fleshings can be made for other applications, their use in AD applications is very attractive due to their high fat content which offers high methane yields. Trimmings (splits and shavings) are somewhat less attractive as a result of lower overall quantities and quality (in terms of organic matter and methane yield), but can still be processed effectively with AD. Fleshings and trimmings have a high content of total solids; therefore, the water to dilute total solids to an acceptable level should come from the wastewater sludge, Table 1.
Hair and some tanned skins are also regarded as biodegradable and have been linked to AD treatment, but their use should be avoided since they can cause more harm than good, especially since better solutions for these materials can be found.
Some concerns have been raised in the past by possible inhibitory effects of certain compounds in the wastewater sludge, in particular the high salinity and the chromium content. However, tests have shown that salinity is not a problem as long as the level is kept below 3,000 mg/l, which can easily be achieved with appropriate handling of raw hides prior to their processing. Furthermore, no real effects of chrome inhibition have been reported for concentrations below 500 mg/l.

Pre-treatment
No special pre-treatment is required apart from the steps that enable free flowing material for pumping and mixing purposes. Consequently, wastewater sludge should not be dehydrated. The fleshings should be hydrolysed while the trimmings should be cut to 2-5 mm (to improve the digestibility of the material), with industrial macerating devices offered by several suppliers.
EU regulation No. 1774/2002 lays down health rules concerning animal by-products and, according to this regulation, fleshings and trimmings may be subjected to anaerobic digestion after pasteurisation at 70°C for 60 minutes with a maximum particle size of 12 mm to ensure sufficient hygienisation and pathogen kill, if the digested sludge is to be spread as a fertiliser.
The different substrates need to be mixed together to form an homogenous mixture that will then be digested. The mixing can be done in the digestion vessel or in a separate mixing tank prior to feeding the mixture to the digestion vessel. Depending on the need, such tanks can have a volume to store enough substrate for several days of operation. In such cases, inevitably hydrolysis of organic matter takes place, which aids the digestion process; however, hydrolysis as a specific pre-treatment is not required.

Operating temperature
There are two possible temperature regimes at which the AD process can be run: mesophilic (around 37°C) and thermophilic (around 55°C). Although the degradation efficiency of substrates is similar at both temperatures, thermophilic operation seems to offer several advantages when it comes to processing tannery wastes. The degradation rate of organic matter is considerably faster, requiring a retention time of around 15 days, which is half of that required for mesophilic operation; consequently the required digestion vessel volume is two times smaller resulting in lower investment costs. Furthermore, fleshings and, to some extent, also trimmings, liquefy at temperatures in excess of 50°C and are, therefore, much easier to suspend in the vessel which allows easier accessibility of the substrates to bacterial digestion. Last but not least, thermophilic conditions also ensure sanitation of the content in a matter of days. There is at least one case in the EU where a thermophilic operated plant has been certified to ensure hygienisation of the substrates and this avoids the need of applying a separate pasteurisation step.
On the other hand thermophilic operating conditions are less robust and much more sensitive to variations in operating conditions. Temperature fluctuations and increase in ammonia concentrations are known to have a detrimental effect on the methane production rate: close control of operating conditions and the input quality and quantity of substrates should be maintained. However, a relatively modern tannery should be no stranger to such industrial control systems, and operating the process efficiently.

Process setup
The conventional way of running an AD process is by employing a continuous stirred-tank reactor with extraction of digested effluent followed by an addition of the same amount of fresh substrates. This takes place several times per day. There exists a maximum organic loading rate at which the process still operates in a stable mode; for biodegradable tannery waste this is between 3.5 and 4.0 kgorg/m3d. However, the organic loading rate is not the sole criteria for determining the volume of the digestion vessel. With decreasing volume the retention time decreases, which decreases the degree of removal or organic matter and consequently the overall methane yield. The final required volume is usually determined by performing a careful cost analysis and considering all economic benefits; this should be done on a case-to-case basis.
Ideally, a digestion vessel should be fed at regular intervals seven days per week. If a tannery does not operate seven days per week then a storage tank should be added in front of the digestion vessel to provide sufficient buffer in terms of the supply of substrates to cover the non-working days when no new waste is being produced. This obviously increases investment costs. As an alternative a sequencing batch process has been considered since it can be coupled directly to waste discharge dynamics from the tannery eliminating the need for a buffer tank. Although such a process performs slightly better in terms of overall methane yield it is problematic when considering heat management and recovery as well as entailing higher investment costs when compared with a convectional semi-continuous process.

Biogas utilisation
Biogas, which contains around 65% of methane, has become the major topic of attention in recent years because of its energy potential and renewable nature. It is a source of revenue for the tannery; therefore careful consideration should be given to finding the optimal way of utilising its energy potential. One option is using it as fuel in a Combined Heat and Power (CHP) unit to produce electricity and heat. Such utilisation involves relatively high investment and operating costs and also yields lower quality heat (hot water at 90°C instead of steam). Since tanneries are large consumers of primary energy sources for production of heat, biogas is best employed in existing facilities to replace part of this fuel and contribute to a reduction of greenhouse gas emissions.

Handling of digested sludge
Technical and economical evaluation studies have been performed and all indicate that digested tannery waste has a definite value as a fertilizer based on its nutrient content. However, this option is usually only possible for vegetable-tanning processes since high chromium content in tannery waste prevents its eventual use as a fertilizer.

Conclusion
The aim of this communication was to provide basic data for a first estimate of the energy potential of biodegradable tannery wastes. It was also to point out the steps involved, and the number technological options that are available for anaerobic digestion of biodegradable tannery wastes. Although the considerations to be taken in account are quite complex, extensive knowledge and experience already exists to enable an optimal solution in a relatively short space of time to be found.