Wastewater Treatment Residuals (WWTR) at Forest Product Mills as Non-Waste Fuels

1. Generation and Management

In 2008, 23 percent of WWTR were used in boile rs as a critical source of energy for mills. Use depends on fuel availability, cost of alternative fuels and the capacity of the boiler to use alternative fuels. For example, if a mill has a surplus of hog fuel/bark, it will not need the makeup energy from other onsite residuals such as WWTR.

Over 90 percent of WWTR ar e processed in some manner to ensure fuel product specification needs are met. T he primary preparati on is dewatering with belt filter or screw presses. This results in the delivery of a material to the boiler th at is ready to burn according to the specifications of the m ill’s biomass boilers which are uniquely designed to handle a wide range of fuel qua lities and moisture contents.

Two types of residuals are generated by mills . Primary residuals are generated on a continuous basis. They are typically conveyed directly from the clarifiers to dewatering equipment, dewatered, mixed with hog fuel (with bins and metered conveyors or in piles with a front end loader) and conveyed to boiler in a continuous or semi-continuous

process.

Secondary WWTR are generat ed depending upon the type of secondary treatment in use. In some instances, the residuals are generated continuously (such as for activated sludge); in others (such as aerated stabiliz ation) residuals are generated far less frequently. For activated sludge systems, like with primary residuals, wastewater is decanted from the secondary clarifiers, and re siduals are removed from the bottom of
the clarifiers and sent di rectly to be dewatered.

When aerated stabilization residual s are burned, they are norma lly staged for a time to
allow free water to drain. This would o ccur on a concrete pad on another surface, in a fabric tube or any other manner that allows free water to drain back to WWT system. Additional dewatering is accomplished with scr ew presses, belt presses or other similar
devices.

When WWTR are not immediately used as fuels, they are stored in piles like other biomass materials such as bark and hog fuel because they have similar Btu and content properties. Because of their similar pr operties, WWTR are blended with other fuels to obtain the optimum overall fuel properties and consistency prior to introduction into the boiler to produce stable combustion condi tions. WWTR, with the other fuels, are delivered to the boiler in the same manner as other biomass fuels pr imarily via conveyor belt, but often using a front end loader. As mentioned, in most instances storage is minimized and the fuel is prepared, conveyed, blended with other fuels and burned in a continuous or semi-continuous process.A typical configuration for management of primary and secon dary residuals is shown on the following diagram.

These facts demonstrate that WWTR destined fo r energy recovery in a mill boiler is not
a solid waste because it is not discarded and is managed to enhance its fuel qualities
and stored and burned like ot her biomass residuals.

2. Reliability in meeting steam de mands of the manufacturing process

Multi-fuel boilers at pulp and paper mills are designed to burn a wide variety of biomassbased materials with different fu el characteristics and moisture s. For example, typically, hog fuel ranges from 35 – 60 percent moisture content. However, during certain times of the year, very wet bark (which can be as high as 70 percent moisture content) is used Mill

Process Equalization

Basin Primary

Clarifiers

Process Wastewate r Secondary

Clarifiers Contact / Aeration

Basins WWTR

Dewatering Thickener

Waste Treatment
Residuals Primary Residuals

Secondary Residualsas a fuel. The moisture content of WWT R when burned is in the same range as biomass hog fuel and similar in terms of biomass content and thus fuel value.

A typical mill with an activated sludge system may have combined primary and secondary WWTR content as follows: Wa ter - ~60%; Wood fiber - 8-20%; other Biomass - 12-16%; Paper fillers - 14-20%;

WWTR are produced at a steady and regular rate at pulp and paper mills as clarifier solids streams and thus are a steady, reliable, and necessary part of the fuel stream. Clarifiers exist for the purpose of removi ng the largely paper fi ber solids from the
wastewater as the first st ep in WWT. As noted ear lier, depending on the type of wastewater treatment proce ss in place, some mills have both primary and secondary
clarifiers. Fiber also carries over to se condary treatment system s where it builds up
with time and is then removed and handled in a similar manner to clarifier solids. Removal of WWTR from secondary treatment systems, polishing p onds and other areas downstream of clarifiers is necessary to maintain treatment system performance and it is scheduled appropriately to achieve this objective. Timing is unique to the system configuration, pulp mill performance and other site specific factors.

Thus boiler operators at mills can plan for th at portion of the Btu needs that comes from WWTR when determining the mix of fuels to us e at any given time (hog fuel, other biomass residuals, or fossil fuels). Relati ve amounts of WWTR, hog fuel and other fuels feeding a given boiler depend on the particular site equipment scenario of the mill: dewatering equipment, WWTR properties, and boiler design.

For all the above reasons, WWTR is a reliable sour ce of fuel. In fact, WWTR is used as the primary fuel (and in some cases as the on ly fuel) in fluidized Bed Combustors (FBC) in the US. In fact, one of the best performi ng boilers used to set the MACT floor for carbon monoxide from FBC burns only WWTRs.

3. Material Specifications

WWTR Btu/lb values range from 3,300 to 9, 500 (on a dry basis) depending on whether it is derived from primary or secondary wastewater treatment, type of pulp and paper mill and the quantity of clay or other inert ma terials present. As noted, dewatering of WWTR is a regular practice to ready the ma terial for combustion. In the rare cases when dewatering does not occur, the WWTR m eets the combustion specifications to efficiently burn in the biomass boiler when mixed with other fuel s. Using additional energy to dewater WWTR becomes counterproducti ve to the energy balance of the mill.4. Off-site WWTR Management

According to AF&PA’s 2008 member surv ey, WWTR were managed in a number of different ways: 23 percent were burned for energy, 16 percent for land application, and 13 percent for other beneficial uses (such as raw materials for other processes – roofing felt, flower pots, potting soil, etc.). 1 Thus, more than half of WWTR are managed for beneficial use. When burned for energy, WWTR are almost entirely managed on site. However, when used for other purposes, WWTR ar e shipped off site usually in trucks. As shown in the diagram above, WWTR ar e generally dewatered prior to management – including when shipped off site to r educe transportation costs and ease the handling
of the material.

1 Of course, the NHSM Rule only addresses whethe r materials are wastes when combusted. Management of WWTR for other purposes is not rele vant to a determination whether they are solid wastes when they are managed as fuel to be burned for energy recovery5. Contaminant Levels in WWTRAs presented to OSW in June, the chart below shows that there is a range of values for select contaminants in WWTR butthe variability ranges from 2 to5 times between the mean and the 90thpercentile confidence interval across the samples(see mercury as example shaded in orange). This is no different than many other materials used as fuel such as coal thatare derived from natural materials which have inherent variability. The level of contaminants in no way affects theperformance characteristicsof WWTR as a fuel.Wastewater Treatment Residuals Select Constituent Variability for Available Test DataBTU(BTU/lb)Hg(mg/Kg)Cl-(mg/Kg)As(mg/Kg)Be (mg/Kg)Cd(mg/Kg)Cr(mg/Kg)Pb(mg/Kg)Mn(mg/Kg)Ni(mg/Kg)Se(mg/Kg)Number of Samples94 82 93 77 53 82 84 77 84 84 55Mean58000.040 361 1.15 0.206 0.376 11.35 4.94 381 23.08 0.177Std. Dev.1548 0.063 661 1.18 0.106 0.3666.96 2.55 291 33.17 0.168T-distribution factor1.296 1.296 1.296 1.296 1.299 1.2961.296 1.296 1.296 1.296 1.29990% ConfidenceInterval7,8070.123 1217 2.68 0.345 0.850 20.37 8.24 758 66.07 0.395