The manufacture of leather requires the consumption of large quantities of water and the subsequent treatment of effluent generated. The French Tanning Federation has placed a lot of emphasis on companies' consumption of water. In addition, CTC has launched a large-scale project with the aim of better controlling waste water from the leather production chain.
The project's objective is to identify treatment technologies which enable the reuse of water, testing them in pilot plants and verifying their technical and economic viability.
Tanneries consume large amounts of water which generates effluent loaded with suspended solids, organic matter, (COD and BOD), nitrogen and salts (chlorides and sulfates etc). These effluents are usually subjected to different treatments:
- Physical-chemical treatment for chromium recovery
- Catalytic oxidation for sulfurs
- Screening for suspended solids
- Biological treatment for organic matter
These treatments do not always allow the recycling of all effluents produced in the leather making process. At the request of the French Tanning Federation (FFTM), CTC launched a project to reuse tannery effluents in order to reduce consumption of water and the quantities of liquid waste.
The chosen technology which combines biological treatment and membrane filtration. This technology has the advantage of:
- being relatively compact (compared to a typical biological treatment using activated sludge)
- guarantees a high level of quality after filtration
Two series of tests were carried out:
- on mixed effluent after leaving the physical-chemical treatment
- on beamhouse effluent
1. Membrane Bioreactor
The combination of biological reactor and ultra-filtration has several advantages:
- It retains the biomass in the reactor, allowing concentrations of 10-15 grammes of biomass per litre to be reached, compared to 5 grammes/litre for traditional activated sludge
- Retention of molecules of more than one micrometer in size inside the biological reactor thus accelerating biodegradation
- Guarantees a certain quality of permeate, particularly in terms of suspended matter.
Excellent performance can be achieved on numerous industrial effluents which require high levels of water consumption (agro-food, chemistry and bio-chemistry and paper-making): COD < 10 mg/l, Suspended matter < 2 mg/ l, N <5mg/l, P < 0.5 mg/l.
Taking this into account, from the last stage of ultrafiltration, the treated water should be pre-treated (filtered) before being introduced to the aerated biological tank where the carbonated, nitrogenous and phosphored pollutants will be eliminated.
Filtration membranes are placed in direct contact with the biomass which is responsible for the biological degradation of the pollutants. These organic membranes (flat or hollow fibres) present a very good mechanical resistance.
Membrane filtration ensures the separation of active sludge and purified water: the permeate is aspirated by a pump; the excess sludge is directly extracted from the biological bath to be dehydrated. In replacing the traditional gravity clarification stage, the membrane separation eliminates the constraints linked to sludge decantation; the process is greatly simplified.
The retention of filtration steps is the key phase of the process: it depends on various automated functions (washing against the current/stream, injection of air and chemical cleaning with bleach, acid and soda).
2. Results of pilot plant tests
2.1 Tests using flat type bioreactor (MBR)
A first type of membrane bioreactor was tested on tannery effluents that had already been desulfured and dechromed.
How the pilot plant works
The pilot consists of a bioreactor with biomass in free culture where the flat type ultra-filtration membranes are laid out. The global biological volume is 100 litres.
The nominal hydraulic capacity of the pilot plant is of 10 litres of effluent (ie 10 litres per hour per m2
) with a feeding concentration of around 2g COD/l.
The average concentration of sludge in the bioreactor was of 10g/l, which is 7.5g/l of MVS (organic biomass from the reactor expressed in volatile matter on the dry matter). The bioreactor thus works with a mass charge (pollution load expressed in COD returned to the biomass as suspended volatile matter) of 0.25kg COD/kg MVS per day.
Results of the tests
After around two months of operation, the pilot presented the following performances in the main parameters analysed:
The biogen elements content of the permeate does not pose limitations on the prospective recycling of the permeate in initial beamhouse operations (soak and pre-soak...)
The chloride content remains high: on average 2g/l (maximum 3.9 g/l) in the permeate.
The content of cationic metals in the permeate is presented in the graph (right). These elements can lead to certain defects in the finished leather (stains etc).
The content of metals analysed (aluminium, chrome, copper, iron) of <1 do not limit the prospects of recycling. The metal which poses the most constraints on finished leather is iron but its concentration is low (0.72 mg/l). The calcium concentration (94mg/l) is slightly high but the total hardness presented was typical of soft water (17°F).
The permeate obtained from the treatment of mixed tannery effluent through a flat membrane bioreactor can be recycled in the beamhouse operations (presoak, soak..)
On the other hand, for the following stages of the process, recycling possibilities are limited, without complementary treatment of the high chloride content. With the aim of reducing ion chloride content, reverse osmosis tests are carried out on the permeate resulting from MBR treatment. The results are presented in the following paragraph.
2.2 Results of reverse osmosis
Objectives of the tests
These tests aim to verify the efficiency of reverse osmosis on the elimination of chlorides in order to recycle the permeate in the latter phases of the beamhouse process.
Osmosis works by a balance of concentrations through a very fine membrane which only allows water molecules to pass through.
In the case of reverse osmosis, a hydrostatic pressure is applied in order to force the water to cross the membrane in one direction to separate concentrate (water charged with ions) and permeate (filtered water).
Putting the tests into practice
These tests have been carried out with a laboratory scale pilot plant. The effluent to be treated is permeate coming out of the MBR.
Pilot plant operation characteristics:
- Volume to be treated: 50.5l
- Operating pressure on the diffusing unit: from 10 to 15 bars max.
- Inlet flow: 500l/h
- Permeate flow: 4l/h average (varying from 0.8 to 7l/h.
Results of the tests:
Good performances were observed on the residual pollution, particularly on the chloride ions, of the effluent to be treated.
The reduction in conductivity has allowed the verification of the retention of ions in the distillate and the reduction of their concentration in the treated permeate.
The concentrations of cationic aluminium ions and the copper are lower than detectable limits. The chrome and iron concentrations are lower than 1mg/l and the residual calcium is 4mg/l for a concentration on entry of 91 mg/l. The residual hardness is of 4°F (entry = 29°F), thus typical of soft water.
The pH remained stable around 7.5 in the distillate and the permeate after the reverse osmosis. Even if the different pollution parameters are concentrated in the concentrate, the hydraulic balance remains low as it is only possible to recover 15% of the permeate compared with 85% of the residual concentrate.
The quality of the osmotic permeate does not present any limiting factors to its recycling in the various stages of beamhouse processing. However, despite some good results in terms of performance of up to 90% of chlorides, the hydraulic balance remains mixed, as only 15% of the initial volume is recovered in the form of the permeate compared to 85% in the form of concentrate which should be eliminated. This could limit the relevance of this outcome.
As an indication, the cost of investment in an osmotic plant with a capacity of 170m3/day is e100,000 with an energy cost in the order of 0.15kW/m3, without including the costs relating to renewal of the membranes.
Furthermore, it should be noted that this treatment complements MBR treatment, where other costs would be incurred.
2.3 Bioreactor tests using tubular type membrane
This membrane reactor was tested on the effluent from beamhouse processes.
The diagram shows the operating process of the pilot plant. The plant consists of a bioreactor where the biomass is placed in free culture, followed by a second container where the ultra-filtration membranes are arranged in fibres called ‘spaghetti' or ‘hollow fibre'. The cutting power of ultrafiltration is of 0.1µm. The global biological volume is of 1,200 l (biological and membrane receptacle).
The ensemble is installed in a tank. The automated pilot has a higher hydraulic capacity than the previous one. It treats around 30 l/h of effluent (which is 15 l/hr per m2
The nourishment is carried out in batches in an aerated container, to prevent degradation of the effluent (volume of the container = 790 l). The effluent is filtered beforehand to 0.5mm to avoid clogging the ultrafiltration membranes.
The beamhouse effluent load, which is very high, (COD around 14-15 g/l), means this effluent cannot be treated directly. A dilution factor of 4-5 is necessary so that the charge can be compatible with the capacity of the pilot plant. After dilution, the concentration in the COD in the container is of 2-3 g/l.
The capacity of the biology presents a mass charge of 0.2kg COD/kg suspended volatile matter for a nominal concentration in biomass of 12g/l suspended matter). The tested effluent also needs prior chemical conditioning.
It has been desulfured by adding hydrogen peroxide. It was neutralised: pH was reduced from 10.25 to 7.8 by adding citric acid. Furthermore, taking into account the presence of tensioactives in the effluent, generating some foam, it has been necessary to add some non bactericide anti-foaming agent.
Results of the tests
After around four months of operation, the pilot showed the following results: the COD content in the permeate coming out of the pilot plant was around 100mg/l, the BOD5 is 3mg/l and the total suspended matter was around 4mg/l.
The global concentration of nitrogen remains unchanged between the permeate and the feeding container (120 mg/l). In contrast, a high elevation of nitrates was noted: 271 mg/l in the permeate for 3mg/l on entry.
These strong nitrate contents could in turn lead to acidification of the permeate. Currently the average pH values are close to neutral (6.9).
Thus, if this were to be implemented at industrial scale, an anoxia zone for denitrification (transformation of the nitrates into atmospheric nitrogen) should be considered.
The concentration values of the biogen elements of the permeate are not limitations to the recycling of the permeate in the beamhouse stage.
The chloride content remains quite high: 3g/l in the permeate for a concentration on entry of 3.3g/l. A new series of tests was carried out (with the effluents less charged in chlorides) in order to determine the incidence of high chloride concentrations which appear to inhibit the development of biomass in the reactor. This concentration is compatible with a recycling of the permeate in the first stages of the beamhouse process (presoak and soak).
In contrast in the following stages of the process, the possibilities of recycling are limited, without complementary treatment to eliminate the chloride ions. In effect, the problems of salt migration could induce stains on the skins.
The content of cationic metals in the permeate was analysed. The values of the metals analysed (aluminium, chrome, copper, iron) were found to have content (<1mg/l) which was not limiting in the context of recycling.
The concentration of calcium and, by consequence, the total hardness, are high. However, only a few results on these parameters were obtained. If the content is confirmed with some new tests, it would be necessary to envisage a softening/desalination of the water before recycling.
The tests on recycling permeate resulting from beamhouse effluent by membrane bioreactor will be conducted on presoak, soak, unhairing and pickle.
3 Conclusion and perspectives
The two series of tests were carried out in the first instance using flat membranes on mixed effluent, then with the hollow fibre membranes on beamhouse effluent, give interesting results on the suspended matter, COD and BOD.
This MBR is a compact procedure, simple to implement, which produces good quality water which conforms to existing regulations. The reuse of treated water which has undergone exhaustive reduction of carbon pollutants and MES appears to be compatible with industrial usage in presoaking, soak, unhairing and pickling.
The content of salts (chlorides, sulfates) remains the limiting factor in the generalisation of this technology for a recycling of the baths. Complementary tests must be carried out for calcium.
Regarding the bacteriology, a study is currently underway to determine the potential of this procedure on the elimination of germs.
A new series of tests should be launched in the near future with the aim of testing the technology on tubular membrane bioreactors on mixed tannery effluent. These new tests should lead to further tests on recycling the permeate for beamhouse operations.
For more information including diagrams and tables, please see the July 2008 edition of Leather International