Zero Discharge Technology for Waste Paper Based Mills
Zero discharge technology for waste paper based mills has been developed by Krofta Engineering to control pollution and depleting water resources. The paper industry is high water consuming industry and the consumption pattern in India was:-
• In 1980 the consumption came down to 250 m3/ton of paper
• In 1990 the consumption came down to 100 m3/ton of paper
• In 2000 the consumption has came down to 50 m3/ton of paper
• In 1970s the water consumption was 400 m3/ton of paper
Comparing to the mills operating in developed countries, the water consumption in Indian paper mills is too high. Krofta Engineering Limited has developed a new technology based on clarification of suspended solids through Sedicell that operates on the principle of Dissolved Air Flotation System and have 30 minutes retention time. It can recover fiber by flotation, fillers and fine tune it by sedimentation. If Sedicell is installed as a primary clarifier in the ETP then the quality of fiber recovered is excellent and can be reused in duplex board & kraft paper mills. In case of writing & printing paper mills if the Sedicell is installed on machine backwater, the problem of stickies has to be taken care of in the pulp mill. However, depending upon the raw material used, the floating fiber and settled fillers can be reused in the process. The clarified water from the Sedicell is stored in holding tank, where it pass a pressure on sand filter and activated carbon filter with a pump of 30 meter head as designed per flow. After this process the clarified water from the activated carbon filter is collected in a holding tank and reused back in the plant and paper machine showers.
Machine Back water clarification
Applications of Sedicell
This machine helps in recovering fiber from the top, by flocculating and collecting it at the top of the Sedicell. This is done with the help of a scoop which collects the fiber and transports it to the collecting tank. The filler material which is added as soap stone or calcium carbonate in Paper making process settles down at the bottom. A scrapper which runs continuously along with the scoop and collects the filler material in the tank which can be reused in the paper making process, saving the soap stone, filler as well as fiber. The clarity of water recovered varies depending upon the usage of chemicals used as flocculant to recover fiber. The clarity of clarified water is attained up to 20 – 100 ppm.
This clarified water when used in the pulp mill has an advantage over other process of fiber recovery. The fiber recovered can either be reused in paper making process or can be squeezed on a belt filter press up to 2 – 25% and the consistency can be used either by the board mills or any other inferior quality of paper. This process increases the brightness of the pulp being manufactured. It is experienced that the recovered fiber in the paper mills using imported waste paper has certain stickies in the fiber. Whereas the mills using Indian waste paper / white paper cuttings or non-stickies imported waste paper, can easily use this fiber from the machine backwater and also for agro pulping based paper machines.
Sedicell is widely used in de-inking plant, where the entire effluent is taken and clarified in Sedicell. The sludge recovered at the top is not at all used in the paper mills because it contains black ink particles and some foreign material which are separated during de-inking process. We recommend belt filter press for squeezing the sludge recovered from the Sedicell and the clarified water having suspended solids of less than 50 ppm should be passed through the pressure sand filter (PSF) and activated carbon filter (ACF) to remove the porous material and the partial color. After the completion of this process, the clarified water can be reused in the pulp mill and for the agro pulping based paper machines, without wasting a single drop.
Replacement of Conventional Clarifier – Saving space and civil construction
Sedicell has replaced the primary and secondary clarifiers for the effluent treatment plant, saving huge civil construction costs & space and the paper quality of effluent being clarified with this equipment. The efficiency of this equipment is 96% as compared to conventional clarifier which is 70 – 80%. The BOD reduction in the Sedicell is 35 – 40% as it operates on the principle of dissolved air flotation system which percolates air in dissolved form in the clarified water, 25% of which is reused for flocculation of aerator effluent. The dissolved air helps in floating fiber at the top with the help of flocculants.
For a flow of 6000 m3/day flow, the conventional clarifier will have dia 18 meters and Sedicell will have dia of 8 meters. Condition of the aeration tank is also improved because of the efficient removal of suspended solids from the raw effluent. The suspended solids present in clarifier can be reused in kraft and duplex paper mills but in case of writing printing papers it has to be disposed off or burned in the boiler.
For paper machine showers water clarification, Krofta Engineering has developed Spray Filter, KS Filter, Pressure Sand Filter & Activated Carbon Filter. The purpose of providing filters after Krofta Sedicell is to reduce the suspended solids upto 20 ppm through KS Filter and PSF. Through the use of Spray Filter we get clarity of less than 38 microns and the clarified water from the Spray Filter can easily be reused in the paper machine showers.
The balanced effluent flow ranging between 10 to 20% is treated through effluent treatment plant based on aerobic process and passed again through Pressure Sand Filter and Activated Carbon Filter to reduce the suspended solids less than 20 ppm, BOD less than 30, COD less than 150. These parameters are required to be maintained by effluent treatment plant, running religiously to achieve the parameters of getting this water treated through RO plant for getting the fresh water characteristics in order to reuse it in the boiler and other areas where fresh water is necessary. The rejects from RO plant is taken on belt filter press. They are either burnt in the boiler (case to case basis), or sold to board mills for being land fills. The filtrate water from the belt filter press can be re-circulated in the system making a zero discharge possible for waste paper industry.