Water is the driving force of pulp and paper mills and, as a result, it is essential to manage incoming and outgoing water streams 24 hours a day. Recycling, optimum use of raw materials and the minimisation of pollution have become increasingly important in a world of dwindling resources.
Sappi is extremely aware of its environmental responsibilities and to this end has contracted Thuthuka Project Managers to manage water treatment projects at four of its mills.
Thuthuka is an established South African company providing turnkey solutions in water treatment, water reclamation, waste reclamation, waste management and related industrial environmental management. The company works in close conjunction with Best Water Technology Africa (BWT Africa), of which Best Water Technology AG is the Austrian-based mother company (based in Europe with 42 subsidiaries and sister companies). BWT Africa designs, manufactures, installs and commissions mobile water treatment package plants. Treatment capacity is always to the client's requirements.
To date, Thuthuka has been involved in projects at the following Sappi mills: Enstra, outside Springs; Adamas, near Port Elizabeth; Saiccor near Umkomaas and Sappi Tugela, outside Mandini. The Enstra mill is regarded as being a flagship project.
A successful plant is a well-planned plant with proper research having been done from the outset. Hennie Cronjé, Projects Engineer at Thuthuka Project Managers, says that Sappi played a major role by cooperating closely with the Thuthuka engineers: "A client is the entry and the exit to a project. If you think of the entry as being the information the client gives as to their treatment needs, Sappi gave us huge amounts of information which in turn allowed us to start adding value by coming up with a proper process design. At the end of the project - the exit - Sappi's excellent ongoing maintenance and proactive operation on the plant has resulted in an extremely successful project, which has now been in operation for five years."
He adds another reason why the Enstra project is regarded as being a major success:
"In terms of optimisation of the plant, we used the existing infrastructure wherever possible. By modifying the existing structures to get to the final aim, we optimised Sappi's investment."
Sappi started investigating the project in 1994, beginning an investigation period of nearly two years. Thuthuka Project Managers was required to treat wastewater discharged from the Sappi Enstra mill, in order to control effluent and reduce pollution when the treated water re-entered the Blesbokspruit. They designed an activated sludge system that was a retrofit of an existing system. The project was the first activated sludge plant of its kind in the country on a bleach plant effluent. Freddy Viljoen, Water and Effluent Superintendent at Sappi Enstra, says: "The structure was already there. Thuthuka converted an old clarifier into the system."
Construction on the plant began in November 1995, and the plant was commissioned in August 1996. Viljoen reports that about 30 million litres of water is discharged daily from the plant of which 4,5 million is treated through the activated sludge plant. Once it has been treated, some of this water is then discharged into the Blesbokspruit, which is a part of the Vaal river system, and the waste is returned to the plant. Viljoen says that before treatment, COD levels of the effluent water are about 1000 parts per million (ppm); after treatment the COD levels are about 250 ppm.
He explains: "The mill produces two types of effluent. One stream is paper machine effluent, which contains lower COD levels and which is treated through an extended aeration lagoon, while the other effluent stream comes from the bleach plant and is highly saline. The plant designed by Thuthuka divides the two streams - they were not separated previously - and the highly saline portion is treated through the activated sludge plant to prevent mixing of the two streams. Since separating the streams, a total of 6 million litres of the paper effluent is recovered daily, which was not possible before, due to the 4,5 million litres of the saline water in the effluent. This resulted in a saving of about 2 million litres of Rand Water intake into the mill.
"This is the first phase of the project. We intend to embark on a second phase at some point in order to remove more dissolved salts. It is already possible through reverse osmosis, but we are looking at changing our bleaching processes to reduce the salt content of our effluent streams and, in so doing, make the waste treatment process more cost-effective."
Step by step, the process is as follows:
Viljoen explains that the water comes in through cooling towers to reduce temperature. It then goes into a balancing pond where calcium carbonate is added for pH correction. "The bleach plant effluent is normally at a low pH and we correct it to about 7, using calcium carbonate which is actually a by-product of the chemical recovery system on the site."
After this, the effluent goes into a flow control box where Sappi is able to control and stabilise flow to the activated sludge plant. Viljoen clarifies: "In the flow control box we also add nutrients for the bacteria in the sludge plant, namely phosphoric acid and urea. This then goes into the activated sludge plant, where the effluent is mixed with the sludge that we remove from the plant - in other words there is always an internal recycling stream."
From the activated sludge plant, the effluent is aerated with compressed air. From there, it flows into a reactor containing five aerators that pump in oxygen. It then flows into a clarifier where the solids are separated from the water. The sludge is then returned to the plant and the treated effluent flows into the river.
The activated sludge plant at Sappi's Enstra mill has helped in improving the quality of the effluent discharged as well as helping to achieve recovery or re-use of effluent in the mill. This reduces the volume of water intake into the mill and thus saves valuable resources.
Thuthuka Project Managers
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