WO1994027912A1 - Method for purifying chlorophenol-containing waters - Google Patents

Abstract

The invention is related to a method for purifying water contaminated by a chlorophenol compounds containing agent, in which method the chlorophenol-containing agent is principally leached along with ground run-off water and rainwater into a waterway and/or adsorbed in the soil, and therefrom further to the groundwater. A characterizing property of the invention is principle of preferentially utilizing the existing processes of a saw mill for the purification process of the chlorophenol-containing waters, whereby investment in new machinery or equipment is not necessarily needed. According to the invention, the purification of water is most advantageously implemented utilizing such auxiliary processes of timber conversion as water-spraying of log piles, intermediate storage of bark and combustion of bark. The steps of the purification method are used and adapted to conform in an appropriate manner with the different steps of the actual manufacturing process and the purification arrangement used.

Description

Method for purifying chlorophenol-containing waters

The present invention relates to a method according to the preamble of claim 1 for purifying chlorophenol-containing waters .

In Finland as well as in other countries of the nordic coniferous wood zone, sawn timber has been preserved against blue-stain caused by different fungi by means of chemical treatments. In last decades, the fungicidal agents in the most widely used preservatives have been various chlorophenol compounds, typically penta-, tetra- and trichlorophenols .

The preservation of wood has been based on the open-tank, log bundle dipping and spraying methods. In the open-tank method, the individual pieces of timber are impregnated in a preservative solution. In the bundle dip method entire bundles of timber are briefly immersed in a tank containing the preservative solution. In the spray method the surface of timber is sprayed with the preservative solution.

Due to insufficient protective measures applied in such preservation methods, particularly the open-tank method and the bundle dip method could lead to significant spills of the preservative solution to the environment. Owing to lack of knowledge and/or negligence, the preservative immersion tanks could be emptied directly into the soil. Also various disturbance or disaster situations could cause significant releases of preservative substances into the surroundings .

The toxic effects of chlorinated phenols on human health were recognized in Finland in the 1960 's and 1970 's, while their environmental risks were understood only as late as in the beginning of the 1980' s. The use of chlorophenol- containing anti-blue-stain preservative chemicals was prohibited in Finland as well as in several other countries in the late 1980 's, while their use is still permitted in some countries .

When spilled on the ground, chlorophenols may be absorbed in the soil or become transported with ground run-off water or rainwater to the waterways or groundwater. In the topmost layers of the ground, chlorophenols undergo a partial decomposition through biological pathways, but in the lower layers below the surface they have turned out to be rather stable. Chlorophenols have been found in saw mill sites even several decades after the termination of timber treatment with anti-blue-stain preservatives in the area.

Chlorinated phenols are transported in waterways along with the water currents and ultimately bound with bottom sediments. In a waterway, chlorinated phenols may become enriched in the human food chain via the microfauna and fish thus posing a health risk. Chlorinated phenols reaching the groundwater present the most complicated case, because they do not undergo natural decomposition there owing to a too low temperature, oxygen supply and bacterial activity. From groundwater chlorophenols can also easily enter the communal water supply networks thus causing a health risk.

In Finland, chlorophenol-containing preservatives effective against blue-stain have been used by an accumulated total of 24,000 metric tons. In the 1970 's, their total annual consumption worldwide has been at an annual level of about 50,000 metric tons. Soil and waters contaminated by chloro¬ phenols have been a hazard particularly characteristic to saw mills and their surroundings. The change of use of the saw mill lots (e.g., development into residential lots) presumes the eradication of the areas free from chloro- phenols. Similarly, the purification of groundwaters, particularly when contaminated by chlorophenols, is considered a necessity owing to the acute or potential health risk involved.

An advantageous composting method has recently been devel- oped for decontaminating chlorophenol-containing soil.

According to said method, contaminated soil is piled into an elongated heap in which beneficial conditions are esta¬ blished for the biological decomposition of chlorophenols . The decomposition process is performed by the native microbes (bacteria, fungi and enzymes produced by them) of the soil . An important precondition is that the environ¬ mental parameters such as temperature, pH, moisture content, nutrients and other factors crucial to the activity of the microbes are adjusted optimal. Neither may the compost incorporate constituents inhibiting the biological decomposition process such as toxic substances.

If the soil contains a zero or insufficiently low concen¬ tration of native bacteria capable of decomposing chloro- phenols, suitable bacterial strains can be grown in separ¬ ate pure or mixed colony cultures and then added to the compost. The biological decomposition process can be further enhanced using various bioreactors or slurrying methods .

Biological decontamination of chlorophenol-containing soil is a most suitable and cost-effective though time-consuming cleaning process for saw mills. Under favorable conditions the concentration of chlorophenols in the soil can be reduced in 2 - 3 years by composting to 1 - 5 % of the initial concentration. The treated soil can later be used as landfill, for landscaping and other similar use.

If the soil is heavily contaminated with chlorophenols, the safest decontamination method is incineration. However, such thermal decomposition is extremely expensive and is available only to a limited capacity. A plurality of different treatment methods are available for purification of chlorophenol-containing water. Water decontamination free from chlorophenols have been done using mostly biological methods in conjunction with, e.g., wastewater treatment plants or other sludge processing facilities. Purification has also been made in separate bioreactors using microbial strains produced in mixed or single-strain colony cultures. Alternatively, chlorophenols can be separated from the water by activated carbon or other filtration methods and then decomposing them through incineration or composting of the filters. Removal of chlorophenols from water has always necessitated the acquisition of a bioreactor, filter unit or similar treatment plant. This requirement elevates the investment and operating costs of water purification to an intolerably high level. Consequently, such approaches have not yet been implemented in full-scale water purification plants, but rather, the available information is based on laboratory and pilot-plant scale investigations.

Biological purification of chlorophenol-containing waters takes place basically in the same fashion as the biological decontamination of contaminated soil. First, such precondi¬ tions (temperature, pH, nutrients, etc.) are established that facilitate the growth of chlorophenols-digesting microbes. When necessary, the purification process is seeded with bacterial strains grown in controlled cultures. Biological purification is most typically carried out in either existing wastewater treatment plants or purpose- built bioreactors. In conventional wastewater treatment plants based on the activated sludge method, however, chlorophenols may cause disturbance to the normal purification process, whereby the usable chlorophenols treatment capacity remains small. In bioreactors the chlorophenols-decomposing bacteria are preferably bound with the bioreactor filler material or the fluidized bed solids to prevent them from leaching out from the reactor along with the treated water. Under favorable conditions the biological water purification process may reach a purification efficiency of 99 %, although the method is extremely susceptible to disturbances in the process.

In the photochemical decomposition process of chloro¬ phenols, the chlorophenol-containing water is subjected to ultraviolet radiation. The UV radiation may be provided by the sun or an artificial source. The photochemical decompo- sition process called photolysis attains an almost complete purification result, while such factors as high water turbidity or dirt content may crucially impair the purifi¬ cation efficiency.

The electrochemical purification process also attains an almost complete decomposition of chlorophenols in the water. The chlorophenol-containing water, which may have a conductivity improving agent such as a salt added to it, is taken to an electrolysis apparatus. The electric current passing through the liquid then accomplishes the decomposi¬ tion of the chlorophenols . The process is controlled by the adjustment of the electric current and voltage and choice of the catalyst used.

Thermal decomposition of chlorophenols through incineration occurs in the same fashion as the incineration of solid waste. The combustion must be performed at a sufficiently high temperature using a sufficient supply of oxygen and adequate retention in the furnace. Then, the result is almost complete decomposition of chlorophenols. The method is costly and therefore only rarely suited for the purifi¬ cation of chlorophenol-containing waters.

Adsorption as such is not an actual purification method for chlorophenols, but rather, it is utilized to bind chloro¬ phenols in a carrier material which is then easier to destroy. The most effective adsorbing materials include, e.g., activated carbons. Chlorophenols adsorbed by such materials can then be decomposed biologically by composting or thermally by incineration.

Under favorable conditions and using dedicated equipment, all above-described methods are suited for the purification of chlorophenol-containing waters. However, such methods and equipment are costly and fairly sensitive to changes in the process conditions. Consequently, some of the above- described methods have in practice failed to meet the requirements set for a satisfactory purification result.

Due to the above-described problems, the threshold for initiating water purification activities has remained high, and so far no full-scale purification project of chloro¬ phenol-containing waters has been started. Particularly in the saw mill industry where the need for removal of chloro¬ phenols is urgent, the high cost of water treatment and unreliable operation of the purification processes have prevented the launching of purification projects.

It is an object of the present invention to overcome the disadvantages of the prior-art methods and to provide a multistep method for purifying waters containing chloro- phenols by virtue of the adsorptive properties of bark.

It is a further object of the present invention to provide a method for advantageously using photolysis, bioreduction, adsorption and thermal decomposition in a distributed purification process of chlorophenol-containing waters resulting from the production steps of the timber and saw mill industries .

Description of the method according to the invention:

According the present method a multistep method is used for purifying chlorophenol-containing waters. The theory of the method according to the invention is described in greater detail in a licenciate thesis by Erkki Salmenlinna under the title "Purification of chlorophenol-containing waters in conjunction with the saw mill processes of timber conversion", Aug. 15, 1993, Koski (H.L.) , Finland. In-depth test were conducted for the thesis to assure the function of the method according to the invention. The test results corroborate the applicability of the invention.

The method according to the present invention attains a satisfactorily good purification result in all cases notwithstanding changes in process conditions, variations in the chlorophenols content of the intake water or other similar disturbing factors. The characterizing properties of the method according to the invention are disclosed in the annexed claims .

In the method according to the invention, purification of chlorophenol-containing water is carried out in parallel with the industrial conversion process of timber. The purification method disclosed herein is comprised of puri¬ fication steps each of which utilizes the conventional process steps of the industrial conversion of timber. The characterizing property of the invention is that the purification process steps are associated with the process steps (partial processes) of timber conversion in a manner that requires no extra investments by the saw mill. The stepwise purification method is adapted to the partial processes appropriately in series or parallel so that any possible disturbance in the actual timber conversion process, its auxiliary processes or the purification process itself have no appreciable effect on the final result of purification.

In conjunction with timber conversion processes, purifica¬ tion of water containing chlorophenols can be achieved without extra equipment utilizing A) photolysis, B) adsorp¬ tion, C) bioreduction, and D) thermal decomposition.

According to the invention, the purification of water is most advantageously implemented utilizing such auxiliary processes of timber conversion as 1) water-spraying of log piles, 2) intermediate storage of bark and 3) combustion of bark.

A normal timber conversion (log sawing) process to which the invention can be applied is as follows:

- The logs are transported with bark from the woodlands to the saw mill. - The logs are stored with bark in piles on the woodyard. The purpose of the log storage is to ensure uninterrupted operation of the saw mill with a suffi¬ cient supply of timber for approx. 1 - 6 months irre¬ spective of any delivery disturbances. - During the warm season, which in the Nordic countries is from May to October, the log piles are kept moist by continuous water-spraying. The purpose of water- spraying is to prevent insects from infecting the wood and thus causing considerable quality damage. - Prior to sawing the logs are debarked. The bark is stored in heaps. Composting in the heaps heats the bark, whereby water contained in the bark is evaporated and the bark loses water. The bark is conventionally used in the power plant of the saw mill as a fuel for generating heat that can be used for drying the wood and heating the premises .

- The logs are sawn into planks and boards . The converted timber is sorted by dimension and quality. Green, freshly sawn timber becomes easily blue-stained by fungi and molds. To prevent this, the saw mill industry has used chemical agents to prevent blue-staining. The most common preservatives employed in the prior art contained chlorophenols .

- Sawn planks and boards are most commonly dried in kilns. Such drying requires external energy, which can be produced in the saw mill power plant using bark as the fuel. The bark is conventionally combusted at a temperature of 900 - 1100 °C, which is advantageous in terms of combustion efficiency and low emissions. Chlorophenols undergo thermal decomposition at tempera- tures greater than 850 °C.

- After kiln drying the timber is packaged and shipped to the customer.

The decomposition of chlorophenols according to the invention is attained in conjunction with the auxiliary processes of the above-described saw mill operation by virtue of the following steps:

1. The chlorophenol-containing water introduced from a source into the decomposition process of chlorophenols is combined with the water circulation used for spray¬ ing the log piles. During the spraying of the water, it is subjected to solar light containing ultraviolet radiation, whereby the chlorophenols are decomposed by a first reduction step of photolysis . In this step the auxiliary process of water-spraying is utilized for the photochemical decomposition, or photolysis, of the chlorophenols contained in the water to be treated by subjecting the water to solar UV radiation. As the spraying water is continuously circulated, photolysis takes place as long as the supply of UV radiation is available.

2. During prolonged circulation the turbidity of the spraying water increases and the efficiency of photolysis decreases. Advantageously, during the water- spraying process a portion of the chlorophenols become adsorbed in the bark or a similar adsorbing material . The adsorption of the chlorophenols in the bark or similar adsorbing material is utilized for the two chlorophenols reduction processes described below, namely bioreduction and thermal decomposition.

3. After the water drips over the piles down to the soil, it returns back to the spraying-water pond. On its way along the soil it meets bacteria or other micro- organisms capable of biologically decomposing chloro¬ phenols, whereby biological reduction, or bioreduction of the chlorophenols occurs in the soil. During the season the logs are sprayed, the conditions for bioreduction are generally favorable as the ambient temperature is sufficiently high for starting the bioreduction process. After getting started, microbial bioreduction will continue even if the ambient temperature would fall below 10 °C.

Also that portion of the chlorophenols bound in the bark undergoes bioreduction as the bark is generally stored in heaps for up to several months prior to its use as a fuel. In the heaps the bark or other absorbent behaves like a compost in which the internal tempera- ture is elevated by composting thus creating optimal conditions for a continued reduction of chlorophenols through bioreduction by naturally occurring bacteria or other microorganisms capable of biologically decomposing chlorophenols.

4. The stored bark is combusted in the saw mill power plant, whereby any chlorophenols remaining over from the bioreduction step are subjected to thermal decompo¬ sition. Complete thermal decomposition of chlorophenols into nontoxic compounds requires a minimum temperature of 850 °C, approx. 3 s retention in the combustion zone and a sufficient supply of oxygen. All these require¬ ments are easily met during the combustion process.

Accordingly, the method described above always attains decomposition of chlorophenols by virtue of a novel method of utilizing the auxiliary processes of timber conversion without needing any extra equipment. Neither are the operating costs of a saw mill facility increased with regard to normal timber conversion. Moreover, the series and parallel connection of the different purification steps according to the invention assure that the decomposition of chlorophenols takes place with a sufficient efficiency even if operating disturbances would occur in the individual auxiliary or purification processes used in the method.

When required, the efficiency of the purification process can be further enhanced by adding more process steps or using proper additives where economically justifiable. E.g., the efficiency of the photolysis step can be improved by arranging a separate photolysis circulation for the "clear" chlorophenol-containing intake water prior to routing it to the more turbid circulation of the log pile spraying water. Thus, the higher effect of photolysis on clear water can be utilized.

Also the bioreduction step can be improved by seeding the bark heap with bacteria or enzymes obtained from separate cultures of microbes with a specifically high digestion capacity of chlorophenols. Also a controlled dosing of nutrients can be used to enhance bioreduction, while the risk of their detrimental effects via releases into waterways or emissions from the combustion step may simultaneously increase.

The invention is next examined with reference to the appended drawings, in which Figure 1 is a flow diagram of the purification process in conjunction with the auxiliary processes of a saw mill .

With reference to Fig. 1, chlorophenol-containing water is routed to a spraying-water pond 1, wherefrom the spraying water is pumped by a pump 2 to spray nozzles 3. The water spray 4 is used for moistening the piles 5 of undebarked logs 5. As the water is subjected to solar UV-radiation 6 in the spraying water pond 1 and the water spray 4, the chlorophenols contained in the water are subjected to photochemical decomposition through photolysis.

The spraying water enters the bark of the logs in the piles 5 and the chlorophenols contained in the water are adsorbed in the bark. This step utilizes the adsorption of the chlorophenols .

From above the log piles 5 the water flows along the ground 7 back into the spraying water pond 1. During this step the microbes of the soil perform biological decomposition of the chlorophenols meeting the microbes. This step utilizes bioreduction by microbes .

After storage in piles the logs are transferred for debark¬ ing to the debarking machine 8. The bark separated from the logs is transferred to bark buffer heaps 9. The heaps can be further soaked with the overflow water 10 from the spraying water pond 1. Composting in the heaps 9 elevates the temperature of the bark and aids the evaporation of excess water from the bark. By arranging a sufficient supply 11 of oxygen into the heaps through, e.g., sub¬ surface drainpipes, the bark heaps can be operated as biological composts in which the bioreduction of the chlorophenols is continued. From the heaps 9 the bark is transported to a power plant 12, where the bark is used for generating heat through combustion. When the combustion is carried out at a temperature greater than 850 °C using sufficient excess oxygen and retention, the chlorophenols still existing bound in the bark are thermally decomposed into nontoxic compounds .

An essential property of the invention is that the purifi- cation of chlorophenol-containing waters is achieved by virtue of utilizing the existing processes of a saw mill, whereby investment in new machinery or equipment is not necessarily needed. The present method does not either require the use of additives such as chemicals-digesting bacteria, enzymes, nutrients or auxiliary chemicals.

Therefore, the investment and operating costs and hence also the threshold for initiating the purification operations remain low. The use of auxiliary equipment or materials is, however, possible in conjunction with the method when considered necessary and advantageous for improving the efficiency of the purification process. To this end, the method can be complemented with, e.g., an electrolysis, filtration or similar step. Moreover, the mutual order of the process steps according to the method can be varied, and some of the steps can be added or removed as required. The purification method can employed in and adapted to the production layout of the saw mill and desired configuration of the purification process using the most appropriate sequence of the purification steps.

While the invention is particularly suited for use in the timber conversion and saw mill industries, the method according to the invention can obviously be used in and applied to any other manufacturing process and facility incorporating suitable partial processes . The method according to the invention is primarily intended for purification of waters contaminated by a chlorophenol compound, whereby said waters can be routed to the purification process principally from outside the plant, e.g., from a contaminated waterway, aquifer or similar source. If the chlorophenol-containing chemical is used in the plant itself, the method according to the invention can as well be used for the purification of the in-plant chlorophenol-containing process water or similar contaminated water.

For those versed in the art it is obvious that the inven¬ tion is not limited by the exemplifying embodiment described above, but rather, can be varied within the scope and spirit of the annexed claims.

Claims

Claims :

1. A method for purifying water contaminated by a chloro- phenol compounds containing agent, in which method the chlorophenol-containing agent is principally leached along with ground run-off water and rainwater into a waterway and/or adsorbed in the soil, and therefrom further to the groundwater wherefrom the water is further taken to a purification process, c h a r a c t e r i z e d in that the water contaminated with the chlorophenol compounds is appropriately collected to a spraying water pond (1) , wherefrom it is further sprayed onto piles (5) of undebarked logs or similar objects on which the water is adsorbed in the bark or similar material as well as the soil (7) under the piles (5) , wherefrom the water is routed back to the spraying-water pond (1) , and that after being subjected to at least one moistening cycle the log is debarked and the bark is advantageously stored in buffer storage heaps (9), from which heaps (9) the bark is transported to a power plant or similar incineration facility and subjected to combustion (12) there.

2. A method as defined in claim 1, c h a r a c t e r ¬ i z e d in that the circulation of the chlorophenol- containing water used for moistening the logs advantageous¬ ly is continuous .

3. A method as defined in any foregoing claim, c h a r ¬ a c t e r i z e d in that the moistening water is advanta- geously routed from the moistened log pile to another log pile not yet treated by moistening.

4. A method as defined in any foregoing claim, c h a r ¬ a c t e r i z e d in that the moistening of the log pile (5) with the chlorophenol-containing water is advantageously performed by spraying.

5. A method as defined in any foregoing claim, c h a r ¬ a c t e r i z e d in that air (11) is routed if necessary into the buffer storage heaps (9) the ensure a sufficient supply of oxygen.

6. A method as defined in any foregoing claim, c h a r ¬ a c t e r i z e d in that chlorophenol-containing water (10) taken from the moistening water pond (1) is adsorbed into the buffer storage heaps (9) .

7. A method as defined in any foregoing claim, c h a r ¬ a c t e r i z e d in that the chlorophenol-containing water is first introduced into the purified water circula¬ tion prior to being taken to the moistening water circulation.

8. A method as defined in any foregoing claim, c h a r ¬ a c t e r i z e d in that the chlorophenol-containing chemical agent is an anti-blue-stain preservative employed in the timber conversion and saw mill industries parti¬ cularly such as one containing penta-, tetra- and/or tri- chlorophenols .

9. A method as defined in any foregoing claim, c h a r - a c t e r i z e d in that, in conjunction with the partial processes of the timber conversion and saw mill industries, the water purification steps used for the treatment of chlorophenol-containing waters advantageously are photo¬ lysis, adsorption, bioreduction and thermal decomposition.

10. A method as defined in any foregoing claim, c h a r ¬ a c t e r i z e d in that when necessary the method is complemented by introducing chemicals-decomposing bacteria, enzymes, chemicals and/or nutrients into the process.

11. A method as defined in any foregoing claim, c h a r ¬ a c t e r i z e d in that the method is complemented by an electrolysis and/or filtration step.

12. Use of photolysis, adsorption, bioreduction and incineration in either series or parallel connected steps to the end of purifying chlorophenol-containing waters.

13. Use as defined in claim 12 in conjunction with the partial processes of the timber conversion and saw mill industries .