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Dredging Up The Dirt

Pulp and paper companies generate waste from their operations that is typically comprised of a mixture of liquid and solid materials. This sludge is pumped into onsite water treatment lagoons in order...


October 1, 2009
By Pulp & Paper Canada

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Pulp and paper companies generate waste from their operations that is typically comprised of a mixture of liquid and solid materials. This sludge is pumped into onsite water treatment lagoons in order to allow for the separation of water and solids.

Such water treatment has a low initial capital cost but requires ongoing maintenance to remove and dewater the precipitated solids. Over time, solids build up and reduce the space available in lagoons and the efficiency of the water treatment operation. Therefore, lagoons require periodic maintenance to remove and dewater the solids. The dewatered solids can then be properly disposed of and the water returned to the lagoon for reuse.

Go it alone or hire an experienced partner?

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The maintenance of these lagoons requires a number of considerations, the first of which is whether a company wants to manage each step of the process on its own or outsource the work to a qualified service provider. Not only do many pulp and paper mills not have the capital and human resources to execute this maintenance internally, they often prefer not to take on the considerable risk involved.

Effective dewatering minimizes disposal

One of the most cost-effective and efficient maintenance options for pulp and paper lagoons involves dredging the lagoon to remove the sludge and then dewatering the sludge prior to disposal.

The goal through each step is to ensure optimal dewatering results to maximize the water content recovered and returned to the lagoon, and to minimize the volume of solids required for disposal. This is where employing an experienced service provider can be invaluable, by reducing costs associated with transport and disposal of waste.

The key elements involved in a typical project are sludge analysis, dredging, dewatering, disposal.

Sludge Analysis Dictates Best Way To Separate Solids

To ensure optimal results, extensive analytical testing of sludge samples prior to beginning a project is key to determining the right polymer chemical mixture to separate the solids from the liquids. Also, sample testing helps determine the type and size of centrifuge required and the anticipated outcomes.

“This upfront analysis allows us to determine, with a high degree of accuracy, the amount and type of polymer required, and the concentration or dryness, quantity, and characteristic make-up of the resulting solids,” says Ron Keenan, regional manager of onsite services for Newalta in Quebec. “This means no surprises for the customer -they know ahead of time exactly how long the service provider will be on site, how the solids will be managed and how much the project will cost.”

Dredging -manned or unmanned?

In some cases lagoons can be temporarily removed from service so that water can be pumped off and sludge exposed to the elements to dewater naturally over time. However, most lagoons are critical to a pulp and paper mill’s operation so dredging is typically the preferred method to remove solids that build up. In this case, the lagoon stays online and the company can continue to operate without interruption.

A dredge is essentially a floating pump that transfers the in-situ sludge to onshore dewatering processing equipment. There are two types of dredges: manned and unmanned. Unmanned dredges are typically used in hazardous environments where hydrogen sulfide (H2S) or other hazards exist. While safer, unmanned dredges tend to have reduced pump capacity and therefore are less efficient. Manned dredges are more complex, have a larger pumping capacity, and require skilled onboard operators to manage the dredging process.

“Manned or unmanned, skilled dredge operators are key to ensuring safe, efficient dredge operation with no risk to lagoon aeration systems,” says Keenan.

The sludge is removed from the lagoon or pond and chemicals are added to the sludge before the mixture is separated.

Centrifuge-based dewatering most efficient

There are a number of ways to dewater sludge, including beach drying, the use of “geo-tubes,” filter presses, and centrifugation. Centrifugation is the most cost-effective and efficient method. Centrifuges use centrifugal force to separate the solids from the liquids prior to disposal. To increase efficiency, polymer chemicals are added to the sludge prior to being centrifuged.

“Polymers work to bind the suspended particles in the sludge which facilitates and enhances the separation of solids and water,” says Mark McMillan, regional manager of onsite services for Newalta in Western Canada. “Choosing the right polymer is essential. Every lagoon or pond is different and in order to achieve the driest solids, which is what we’re after, you have to have get the mixture just right.”

Polymers are added in such low concentrations that they do not affect or change the characteristic make-up of the resulting solids after centrifugation. Therefore, the solids require no additional processing or treatment prior to disposal to manage the addition of polymers.

There are many different types of decanter centrifuges that can be employed to complete the task of separating liquids from solids. The size and type of decanter centrifuge must be matched to the pumping capacity of the dredge and the nature of the solids involved. Easy- to-separate solids are processed through a large bowl with little retention time (the amount of time the material spends in the centrifuge), whereas hard-to-separate solids with smaller particles require bowl designs with increased retention time to optimize separation.

It is critical in the planning stages to profile the solids to ensure the appropriate centrifuge design is applied to the dewatering process with a supporting chemical program to enhance separation. Many providers of dewatering services have only one centrifuge and employ a one-size-fits-all approach, which can result in inefficiencies. When choosing a service provider, pulp and paper companies should take into account not only the experience, depth and breadth of the operating personnel, but also the equipment being used to ensure the most effective solution possible.

Liquids and solids go their separate ways

After going through the dewatering process, the solids are transported by a series of conveyors and either stored on site or loaded directly onto sealed transportation units for shipment to an appropriate disposal facility in accordance with all regulatory requirements. The treated liquids are usually returned to the lagoon and ultimately reused in the mill’s process operations.

Projects are generally priced in one of two ways: using a per-day rate or based on the total volume of material disposed per “bone dry tonne” (BDT). BDT is the measure of how dry the solids are — the weight of the dewatered solids minus measured moisture content. Mobilization and demobilization of the process equipment are billed in addition to the BDT cost. With day-rate pricing, the customer pays a fixed amount for the equipment and manpower.

Safety considerations

In addition to finding a partner with the operational expertise and the right equipment to get the job done, the environmental, health, and safety aspects of the project are an important consideration.

“A safe, environmentally secure work site is our number one priority on every job,” says Keenan. “Our priority is to ensure the safety of our people and those around the project site, while also preserving the integrity and cleanliness of the customer’s site and eliminating any risk of soil contamination.”

Experience counts

“While the process involved in dredging and dewatering these lagoons sounds fairly straightforward, every project is unique, involving varying specifications of sludge,” says McMillan. “The importance of skilled operators of both dredges and centrifuges cannot be underestimated. This experience enables us to adapt qu
ickly to changing circumstances throughout a project to ensure the job is completed in a timely and cost-effective manner with as little impact as possible for the customer.”

Partnering with an experienced service provider involves additional costs, but significantly lessens risk for the pulp and paper company and allows it to focus on its primary business. Reputable waste management and environmental service companies providing lagoon processing services should have strong environment, health and safety credentials, extensive experience with these types of projects, a range of required equipment, and should be able to complete the task without affecting ongoing operations.

PPC

Wayne Clark is corporate accounts manager, Western Canada, and Luc Robinson is regional accounts manager, Quebec, for Newalta Corporation. Newalta is a national industrial waste management and environmental services company with extensive experience in the pulp and paper industry.

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Every Project is Unique: Hinton and Espanola

The highly effective process Newalta employs at customer sites to service water treatment lagoons has been refined over many years.

Newalta has partnered with a number of companies in Quebec and northern Ontario on dredging and dewatering projects. In March 2009, the company completed a job with the Domtar Espanola mill in Ontario. Newalta used a large manned dredge and two large bowl decanters to remove 2,875 BDT from 35,000 cubic metres of sludge. Nearly 80% of the water was returned to the lagoon. These results allowed the Espanola mill to reduce the environmental impact of this work by reducing the amount of transport required to dispose of the solids.

Newalta has also successfully completed several projects in western Canada, including one in July and August 2009 for West Fraser Timber’s Hinton, Alta., pulp mill. The project involved manned dredging on a lagoon containing both aerobic biological waste generated from the plant’s operations, as well as the town’s municipal effluent. West Fraser has an agreement to accept the town’s effluent into its lagoon and this additional waste needed to be properly analyzed and treated. Newalta successfully processed and removed 6,000 BDT of sludge and returned the water to West Fraser’s lagoon for reuse.