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Anaerobic Waste Water Treatment Process

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Pilot Site


Waste water treatment in a paper mill with integrated mechanical pulping mill, KotkaMills Oy

KotkaMills Oy is the other pilot site in the Life project where VOC emission measurements and pilot tests of the three abatement technologies will be carried out. KotkaMills has a combination of aerobic and anaerobic biological waste water treatment.

Kotkamills is a forest products company in Finland. The company specializes in laminating paper, matt coated bulky paper and sawn products. Kotkamills has two production plants in Finland and a subsidiary, LP Pacific Films Sdn, in Malaysia. The annual production capacity of Kotkamills is 200.000 tonnes of Absorbex Kraft Paper and 40.000 tonnes of Imprex products,180.000 tonnes of Solaris paper, and 230.000 m3 of sawn timber.

Kotkamills produces chemical pulp used in paper production from sawdust. Collected cardboard and paperboard (RCF) will replace some of the softwood sawdust. TMP plant uses also sawmill chips as raw material. TMP is a furnice in bulky paper production. Peroxide/hydrosulfite is used in the bleaching.

Figure 1.1 Kotkamills Oy in Finland.

Description of the waste water treatment system

Kotkamills has both the aerobic and anaerobic waste water treatment systems. Only a small amount, circa 10% of all waste water flow will be treated anaerobically. The total waste water amount is about 21.800m3/day.

Waste waters from mills with high COD-concentration are collected to the primary flotation. Suspended solids are removed with the air flotation and the clarificate is led to a tank where also concentrates from evaporation plant and other clarificates from the mill are led. The anaerobic treatment can be used because of the high COD-concentration in the waste waters. Waste water will be cooled in the two cooling towers suitable for biological treatment (about 37 °C).

In the anaerobic acid stage hydrolytic microorganisms degrade polymer type materials such as polysaccharides and proteins to monomers. Monomers are then converted into fatty acids (VFA) with a small amount of H2. The primary acids are acetic, propionic and buturic with small quantities of valeric. In the acidification stage a minimal reduction of COD occurs. In methane stage the organic acids are breaking down with methane bacteria to methane and carbondioxide in mesofilic range (T=29-38 C,pH=7). In this upflow the anaerobic sludge blanket (UASB) process waste water is directed to the bottom of the reactor tank where it must be distributed uniformly. The waste water flows upwards trough the blanket of biologically formed granules which consumes the waste as it passes through the blanket.

Methane and carbon dioxide gas will rise and are captured in the gas dome. Liquid passes into the settling portion of the reactor tank where solid-liquid separation takes place. COD reduction should be about 80 %.

Anaerobically purified waste water will be led to the aerated basin (1500 m3) together with other waste waters from paper and pulp mills via primary clarifier (see Figure 3).  The aeration basin is a part of the active sludge process, where the aerobic bacterias will use organic material and they will increase. As a result sludge is formed.

After the aeration basin the purified waste water will be led to the secondary sedimentation where biosludge will be thickened in the gravity thickener. All sludges are dryed with presses into 30-35 %. The purified water is led to the sea.



Figure 1.2 Anaerobic reactor.



Figure 1.3 Aerated basin.

Project Actions in Summer 2012


Three different abatement pilot systems (UV-filtration, biofiltration and catalytic incineration) are tested at the waste water treatment plant of Kotkamills. The pilot tests started in May and will be finished by the end of August.

Test results of VOC abatement measurements at Kotkamills

 
Results from the Formia catalytic incinerator pilot tests at the anaerobic wastewater treatment plant

Pilot tests with the three abatement systems were carried out in summer 2012 at the anaerobic wastewater treatment plant. Two pilot sites were decided to be tested, the anaerobic reactor and the aeration basin. The conditions at these two sites vary so much that more valuable information was needed to see the operational properties with different parameters and compounds. All operational data from both tests places was recorded by PLC data logger of the incinerator.


Figure 3.1 Catalytic incinerator pilot tests at Kotkamills in 2012.

Figure 3.2 Catalytic incinerator pilot tests at Kotkamills in 2012.

The cleaning efficiency rates of the incinerator varied between 87-97%. The changes in the purification rated were caused because of the great fluctuation in VOC concentrations and because of existing methane. Based on the results there were also differences between the catalysts that were tested (Palladium and Platine). Especially the difference was in non-methane VOC purification. Pt-only catalyst was a better than Pt+Pd catalyst for non-methane VOC purification at used temperatures. Above mentioned maximum VOC conversion rate 97% was achieved with Pt-only catalyst and the minimum 87% was achieved with Pt+Pd catalyst. Part of better performance could be caused by higher inlet concentration during Pt only tests. The higher inlet concentration demonstrated the better purification due to stronger driving force of VOC into catalyst pores.

The results demonstrate that the catalytic incinerator is the technically feasible, cost-efficient system to abate the VOC emissions and eliminate the odour problem at the wastewater treatment process in mechanical pulping.

Results from biofiltration pilot tests at the anaerobic wastewater treatment plant in Kotkamills

Two pilot biofilters were investigated at the anaerobic and aerobic wastewater treatment plant of Kotkamills in Finland in May-August 2012.

 

Figure 3.3 Biofiler pilot plants at Kotkamills pilot site in 2012.

Figure 3.4 Biofiler pilot plants at Kotkamills pilot site in 2012.

The VOC concentration in wastewaters and therefore also in the emissions of exhaust gases was varying heavily. The detected VOC compounds were mainly alcohols and terpenes originating from wood. Unexpectedly, also methane emissions from the wastewater were detected also at the aeration basin as well.

In several references of biofilter applications better cleaning rates have been achieved but biofilter is in general very sensitive to the conditions. Especially in those conditions a prefilter as stabilizing system is needed because of high load of sulphide compounds. On the other hand terpene compounds are not very feasible for biofiltration causing extra need for biobed volume as well.

The cleaning efficiency rate of biofilters was an average 65-70% at the aeration basin. One has to realise that the air flow is remarkably lower than in the case of aerobic cleaning system and the actual cleaning efficiency can be improved with an efficient spraying type pre filter system.

The test results proved that no extra pre-filter e.g. neutralization of the inlet gas is not needed for biofilters. Maximum specific air flow of 50 m³/h per square metre of filter area can not be exceeded and the minimum depth of one metre of filter material is needed to reach an acceptable 80 % cleaning efficiency.

Results from Desinfinator UV-filtration pilot tests at the anaerobic wastewater treatment plant in Kotkamills

Pilot tests were carried out at the anaerobic reactor and at aerobic basin in Kotkamills wastewater treatment plant in June-August 2012. The results demonstrated the cleaning efficiency rates of between 20-30%. The highest rates were received with active carbon filters, but the use of these filters under these humid conditions is not realistic but the tests were carried out for the leaning purposes. The differences of VOC concentration levels were not to be seen in cleaning rate as a fluctuation. Based on the results it can be assumed that the UV filtration technology is technically feasible but works best with lower VOC concentrations as demonstrated with the test results from Stora Enso Anjala Mill.

 

Figure 3.5 Results of the pilot tests in Kotkamills pilot site.

Figure 3.6 UV-filtration prototype.

Figure 3.7 UV-filtration prototype.