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 boilers 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 are processed in some manner to ensure fuel product 

specification needs are met. The primary preparation is dewatering with belt filter or 

screw presses. This results in the delivery of a material to the boiler that is ready to burn 

according to the specifications of the mill’s biomass boilers which are uniquely designed 

to handle a wide range of fuel qualities 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 generated 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 stabilization) residuals are generated far less 

frequently. For activated sludge systems, like with primary residuals, wastewater is 

decanted from the secondary clarifiers, and residuals are removed from the bottom of 

the clarifiers and sent directly to be dewatered. 

When aerated stabilization residuals are burned, they are normally staged for a time to 

allow free water to drain. This would occur 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 screw 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 properties, 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 conditions. WWTR, with the other fuels, are 

delivered to the boiler in the same manner as other biomass fuels primarily 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 secondary residuals is shown on 

the following diagram. 

These facts demonstrate that WWTR destined for 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 other biomass residuals. 

2. Reliability in meeting steam demands of the manufacturing process 

Multi-fuel boilers at pulp and paper mills are designed to burn a wide variety of biomassbased materials with different fuel characteristics and moistures. 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 Wastewater

Secondary 

Clarifiers 

Contact / Aeration 

Basins 

WWTR 

Dewatering 

Thickener 

Waste Treatment 

Residuals

Primary Residuals 

Secondary Residuals 
as a fuel. The moisture content of WWTR 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: Water - ~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 removing the largely paper fiber solids from the 

wastewater as the first step in WWT. As noted earlier, depending on the type of 

wastewater treatment process in place, some mills have both primary and secondary 

clarifiers. Fiber also carries over to secondary treatment systems 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 ponds 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 that portion of the Btu needs that comes from 

WWTR when determining the mix of fuels to use at any given time (hog fuel, other 

biomass residuals, or fossil fuels). Relative 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 source of fuel. In fact, WWTR is used as 

the primary fuel (and in some cases as the only fuel) in fluidized Bed Combustors (FBC) 

in the US. In fact, one of the best performing 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 materials present. As noted, dewatering of 

WWTR is a regular practice to ready the material for combustion. In the rare cases 

when dewatering does not occur, the WWTR meets the combustion specifications to 

efficiently burn in the biomass boiler when mixed with other fuels. Using additional 

energy to dewater WWTR becomes counterproductive to the energy balance of the mill. 
4. Off-site WWTR Management 

According to AF&PA’s 2008 member survey, 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 are shipped off site usually in trucks. 

As shown in the diagram above, WWTR are generally dewatered prior to management 

– including when shipped off site to reduce transportation costs and ease the handling 

of the material. 

                                                      

1 Of course, the NHSM Rule only addresses whether materials are wastes when combusted. 

Management of WWTR for other purposes is not relevant to a determination whether they are solid 

wastes when they are managed as fuel to be burned for energy recovery
5. Contaminant Levels in WWTR 

As presented to OSW in June, the chart below shows that there is a range of values for select contamthe variability ranges from 2 to 5 times between the mean and the 90th percentile confidence interval a(see mercury as example shaded in orange). This is no different than many other materials used as fuare derived from natural materials which have inherent variability. The level of contaminants in no way performance characteristics of WWTR as a fuel. 

Wastewater Treatment Residuals Select Constituent Variability for Available Test Da

BTU 

(BTU/lb) 

Hg 

(mg/Kg)

Cl- 

(mg/Kg)

As 

(mg/Kg) Be (mg/Kg) Cd (mg/Kg) Cr (mg/Kg) Pb (mg/Kg) 

Number of Samples 94 82 93 77 53 82 84 77 

Mean 5800 0.040 361 1.15 0.206 0.376 11.35 4.94 

Std. Dev. 1548 0.063 661 1.18 0.106 0.366 6.96 2.55 

T-distribution factor 1.296 1.296 1.296 1.296 1.299 1.296 1.296 1.296 

90% Confidence 

Interval 7,807 0.123 1217 2.68 0.345 0.850 20.37 8.24