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Aerobic Waste Water Treatment Process

Contents

Pilot Site


At the Stora Enso Anjalankoski Mill PGW (pressure groundwood) and TMP (thermomechanical) pulps are produced (Figure 1.). 100 % of the raw material is spruce. The wood comes mainly from Finnish forests. The annual production capacity is 435.000 t/a paper + 215.000 t/a board. The pulp is used to produce paper on two paper machines and carton board on one board machine. The main products are book paper, improved newsprint grades and folding box board.

Wood logs are used in PGW plant  and chips in TMP plant. In both plants pulp is bleached. All waste waters from the paper mill, board mill and mechanical pulping including PGW and TMP production as well debarking are collected to the equalization station. Total waste water amount is about 30.000m3/d. Solid suspension is settled in primary clarifier and the waste waters are cooled in cooling tower to 37°C. The process flow-chart is illustrated in Figure 2.

Nutrients, phosphorus and nitrogen will be added, and waters are pumped to the biological waste water treatment. The objective of this process is to remove soluble and insoluble organics from waste water stream and to convert the material into flocculant microbial suspension that is readily settleable and will permit the use of gravitational solids liquid separation techniques.

The process used in this case is so called nutrient restricted MBBR/activated sludge process. The easily degradable organic materials will be degraded in MBB-reactors (Moving Bed Biofilm Reactors). The amount of organic material in waste waters is described with the biological oxygen demand BOD7.

In aerated reactors the microbes are cleaning the waste water by using the soluble materials as nutrition and consuming oxygen. Additional nutrients (like phosphorus and nitrogen) are needed. The yield will be carbon dioxide, water, heat and new microbes. Microbes in activated sludge will be returned after the final sedimentation to the aeration basin, and the excess sludge will be transferred to the sludge handling.

Process conditions are restricted: pH about 7, temperature under 40°C. After the final sedimentation clarification, the cleaned water is led to the river Kymijoki. The tertiary treatment can be used in extreme situations. The excess sludge from the biological waste water handling and other sludges are mixed, dewatered, dried and incinerated in the boiler house.

The process phases to be piloted with the three VOC abatement systems were chosen based on the results of the VOC emission measurements from the first VOCless Pulping Life project (LIFE06 ENV/FI/201) and the analysed water samples gathered in summer 2011.

Figure 1.1 Stora Enso Anjalankoski Mill.



Figure 1.2 Process description of the aerobic waste water treatment plant at the Stora Enso Anjalankoski Mill, Finland.

Project Actions in Summer 2011

 

Three different abatement pilot systems namely UV-filtration, biofiltration and catalytic incineration were tested at the waste water treatment plant of Stora Enso Anjalankoski Mill. Each plant was tested at the cooling towers and the moving bed biofilm reactor (MBBR) pilot sites (see the process flow-chart, Figure 2).

The idea was to test the operation of the abatement systems and to measure the cleaning efficiency from each of the small-scale pilot plants at least for the two-week period of time. VOC emission measurements were carried out before the start of each test period and after two weeks, at the end of the test period.

During the test periods the abatement systems were controlled by online data recording system in case of malfunctions and possible breakdowns. Also regular visits were made by Meehanite Technology and Stora Enso personnel to ensure the continuous operation of the pilot plants.

The pilot tests started in June and were finished by the end of August. Final VOC measurements were carried out in late October 2011. The measurement results will be analysed and evaluated by the end of 2011.

Measurement Results


VOC emission and effluent measurements were carried out at aerobic waste water treatment plant of Stora Enso Anjalankoski Mill in June - August in 2011. Several measurements and samples were gathered and analysed. The VOC measurements took more effort and time than originally planned. More measurements were carried out as planned because of the great variation in the VOC emissions and effluents. Below the results of the measurements are presented and the final measurement report.

In Anjalankoski Mill waste water treatment system receives water from ten different points from the mechanical pulping process, paper mill and board mill. The first phases of the waste water treatment are the clarifying and cooling. After waste water has been cooled to about 37°C it will be pumped into the aerobic biological treatment. Flow-chart of the waste water process is presented below in Figure 3.1.



Figure 3.1 Waste water treatment process flow.

VOC cleaning efficiency measurements of the three piloted abatement systems were carried out at the cooling tower and at the moving bed bioreactors (MBBR, figure 3.2). Pilot sites represented the most powerful emission sources, which were decided based on the preliminary measurement results carried out in 2007 and 2011.

Picture 3.2 Piloted abatement systems and VOC emission measurements at the moving bed biofilm reactor of waste water treatment plant in Stora Enso Anjalankoski Mill in summer 2011.

Measurement results:

VOC-concentrations of waste water

VOC-concentrations varied greatly specially in the wastewater from the debarking, where the fluctuation in different dates may exceed to the ratio of 1 to 10.  The fluctuation was also great in the wastewaters from TMP (thermomechanical pulping). The concentration level was descending after the cooling and also the fluctuation decreases but the distinct fluctuation is still observed. This has also an effect on VOC emissions.

Detected VOC-compounds were mainly alcohols (ethanol and methanol) and some samples contained also terpens. Additionally the samples contained several unidentified hydrocarbons.

In Figures 3.3 and 3.4 VOC effluent results from 2007, 2008 and 2011 are presented. High variations of VOC concentrations can be seen.



Figure 3.3 VOC concentrations of waste waters in 2007, 2008 and 2011 at the waste water treatment plant of Stora Enso Anjalankoski Mill.



Figure 3.4 VOC concentrations of waste waters in 2011 at the waste water treatment plant of Stora Enso Anjalankoski Mill.

VOC- concentrations discharged into atmosphere

VOC-concentration variety of the wastewaters was measured also in the hydrocarbon emission into atmosphere during various measuring dates (Figures 3.5 and 3.6). Especially on the MBBR-site the fluctuation was high on different measuring dates. The fluctuation was at highest threefold.

Detected VOC -compounds were mainly same as existing in waste waters. Additional to alcohols also different terpenes originated from wood was detected.

Unexpectedly emissions also contained quite high concentration of methane. It probably originates in anaerobic bacterial activity. Methane is not considered as VOC compound emission in this research as in European legislation (EU Directive 1999/13/EC).



Figure 3.5 VOC concentrations emitted to air at the cooling tower of the aerobic waste water treatment plant in 2011.



Figure 3.6 VOC concentrations emitted to air at the MBBR basin of the aerobic waste water treatment plant in 2011.

Conclusions

High variation in VOC-concentration of wastewaters and thus also in air in different dates was somewhat unexpected. It was not detected whether the fluctuation was caused by raw materials, changes in TMP-process at the pulp mill or in waste water treatment process itself. It is probably the synergism of two latter of them. Due to the long process delay time of 2.5 days, any confident conclusions can not be made.

The cleaning efficiency rates of VOC emissions of the three piloted systems varied depending on the concentration of different hydrocarbon compounds in the raw gas introduced to the pilot plants. Also the airflow and plant parameters were affecting. The moisture of the processed air introduced also a challenge to the cleaning system function.

More VOC emission measurements were needed as planned in the project application. This was because of high variations in the waste water VOC concentrations. Measurements were carried out between June-October 2011. This caused additional work and laboratory analyse costs to the project budget.

Test Results of the VOC abatement system measurements

Three different abatement systems namely UV-filtration, biofiltration and catalytic incineration were tested at the aerobic waste water treatment plant in June-August 2011. Each plant was tested at the cooling tower and the moving biofilm reactor (MBBR). The idea was to test the operation of the abatement systems and to measure the cleaning efficiency from each of the small-scale pilot plants. VOC emission measurements were carried out before the start of each test period and after two weeks, at the end of the test period.

Biofilter test results

Two pilot biofilters were tested at the aerobic waste water treatment plant of Stora Enso Anjalankoski Mill in Finland. The exterior dimensions of the cylinder shaped pilot biofilters were: the diameter 1,6 m, the height 2,1 m, and the pilot plant air flow was designed for the air flow rate of 70 - 150 m3/h.

Pre-measurements were made and the pilot plants were placed at the cooling tower and the MBBR basin where the highest VOC concentrations appeared. Two small-size biofilter pilot plants were constructed. Different compositions and moisture of bacteria carrier material were tested. The pilot tests began in May and were finished at the end of August.



Figure 4.1 The biofilter pilot plant

VOC concentrations in the incoming gas fluctuated greatly being at highest on the MBBR -site.  An unexpected concentration of methane for aerobic biological wastewater treatment was detected. That presented a challenge to interpreting the measurement results.

The cleaning efficiency rate of biofilters was an average 80 % at the cooling tower. And the cleaning efficiency rate of biofilters at the MBBR basin was about 70 %. Cleaning rates were better with higher initial concentrations, averaging towards 70 to 75 percent. Cleaning rates were probably fluctuating also due to varying composition and texture of bacteria carrier material in the biofilters.

An overall statement of biofilter tests proved a good performance and sustainable working. It is obvious to realise that the biobed material is to be prepared and matured in good time prior the actual exposure for continuous run. Bioreactor material should be always reserved for the loading of an extra filter batch in the case of malfunction in the process.

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 100 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.

Catalytic incineration test results

Catalytic incineration pilot plant was manufactured and tested at aerobic waste water treatment plant. The pilot plant was developed and manufactured by Formia Emissions Control Oy. Pilot plant’s air flow capacity was 700 Nm3/h and it was located inside 10 feet container to ensure easy transportation. Used pilot plant is presented in the picture below.

Reactor of the pilot unit contained heat exchanger and catalyst structures. Above catalyst there were heating resistors to keep proper temperature to ensure complete oxidation of VOCs. Resistors were used only when VOC concentration was below autothermal point. Autothermal point is a minimum inlet VOC concentration where reactor doesn’t need any support energy for oxidation. In this particular case support energy was needed due to so low inlet VOC concentration. Heat exchanger consisted 6 layers of winded and corrugated steel coils. A material of the heat exchanger was acid proof steel due to high humidity and possible sulphur compounds at inlet air. Catalyst, which was corrugated stainless steel foil, is coated with noble metals. All used materials are stainless steel (AISI 304) except centrifugal fan (construction steel), particle filter box (galvanized steel) and heat exchanger (AISI 316L).



Figure 4.2 The catalytic incineration pilot plant

The conversion rates of catalytic incineration pilot plant varied between 71-83 %. Used oxidation temperatures were 380-480oC. The best conversion rates were obtained by using high temperatures (over 400oC) which was detected especially at the MBBR site. One possible reason for that was that the VOC compounds were different from what was expected and also methane was measured in the inlet gases. Due to this discovery AX carried more measurements in November 2011. Methane was not expected to be there and therefore the used catalysts weren’t optimized for methane oxidation. Two different catalysts were tested but no differences in cleaning efficiencies were noticed.

As a conclusion, the test period was successful for catalytic oxidation method. It was proved that VOCs emitted from wastewater treatment plant can be reduced significantly by catalytic oxidation method.

UV-filtration test results

Desinfinator technology is a combination of reactions and the process combines various different phases to reach the maximum cleaning efficiency. By adding special diffusion filters coated by using the latest nanotechnology and adding other air cleaning processes we have taken our products to the next level, making them revolutionary within the field of air cleaning. One main element of our effective process is to control the airflows and the antiseptical surfaces within our processes. By using a special honeycomb we create a laminar airflow and increase the efficiency of the UV-light significantly. Another important function of the UV-radiation is to activate the diffusion filters and start creating radicals. Additionally a huge amount of Ions are created into the air. Our air cleaning technology is capable of removing bacteria, mould, particles, yeast and odors with a rate of close to 100%.

The unit being designed for the Life -project uses the proven Desinfinator-technology customized for the water treatment plant to clean the air from VOCs. For testing purposes the unit was based on a portable platform and it is weather proof. The whole system was built on a trailer to make it mobile.

The main technologies for VOC filtering includes:

  • pre-filtering
  • UV-C –light
  • photocatalysis filters
  • and other supplementary process parts




Figure 4.3 The UV-filtration pilot plant

The measurements showed that the cleaning efficiency rate varied between 50 - 85 % but most commonly it was near 70 %. During the test period in Anjalankoski the challenges caused by the high humidity were noticed. The basic structure of the Desinfinator system remained almost the same during the test period in Anjalankoski. But some additional components were tested to get data for further development work in a long run.

These first pilot tests showed that extremely high humidity levels of the inlet gas needs some extra attention especially when treated by UV-C radiation and photocatalysis filtration as this process includes sensible components. An additional structure is needed to at least collect the flying droplets from the inlet air. This enables better reliability, performance and the need for service will be less.

Lower humidity levels will solve the reliability issue with the gas mass flow sensor. Also, a different type of a sensor can be sourced or designed to allow constant data flow. Pressure differential sensor is another component to be improved.

According to our test results in Anjalankoski and some additional testing, VOC cleaning rates can be higher. Higher affectivity of the photocatalysis process will be the key to the better cleaning rate. Photocatalysis can be improved in multiple ways in this system in question.

The Anjalankoski period was a great testing and learning process. With the gained results further development work has a great starting point and database to make a step forward in the UV-filtration technology.