Pulp & Paper Industry
Activated sludge yield reduction by the low sludge production process
Afive-month laboratory-scale study wasconducted
to evaluate the feasibility of the low sludge pro duction (LSP)process to treat effluent(of about 800 mg/L total COD) from the Eraser Papers (Nexfor) bleached hardwood Kraft pulp mill, located in Thurso, Quebec. The cost for dewatering and disposing of mixed sludges (consisting of mainly fibrous material from primary treat
ment and biological solids from secondary treatment)is typi cally one of the most significant elements for effluent treat ment (Kenny et ai, 2001). Sludge dewatering costs can increase at a given mill as the ratio of biological solids in the mixed sludge increases, i.e. as good fibre is better-re tained in the pulp and paper mill, and as incremental pro
Table 1 Periods of lab-scale experimentation Period
Days
HRTbl
Experimental notes
(h) Startup 1
1-31
na
First start-up
Reference
39-63
na
Settler slow mixing added. Reference period
LSP1
64-74
4
Wall growth problems
LSP2
75-84
4
Addition of 4 mm plastic pearls into bacterial stage; poor settling of predatory biomass
LSP3
85-100
2
Shorter HRTb
duction increases result in more effluent BOD^ load and
Startup 2
132-139
3
Second start-up
biological sludge production.
LSP4
140-145
3
Stainless steel beaker for
bacterial reactor resulting in wall growth problems
One emerging technology alternative to address this is sue is the low sludge production (LSP) process.
The LSP process requires that a low hydraulic retention time bacterial stage without sludge recirculation be installed, favoring growth on rapidly biodegradable organic matter, typically located upstream of an activated sludge (or biofiltration) process which then acts as a predatory stage. This concept was first proposed and tested on pulp and pa per waste waters at the laboratory and pilot-scales with sludge production varying between 0.0Ig and 0.23g TSS/g COD removed depending on the wastewater treated and process operating conditions (Lee and Welander, 1996). Using a membrane bioreactor for biomass retention instead of an
Bacterial staga
Activated sludge/ predatory stage
Waste
(Sy^ r—
Final effluent
LSP5
146-175
3
Glass beaker for bacterial reactor
Notes: HRTb:(hydraulic retention of the bacterial stage); na: (not applicable). Day 1 was September 1, 2000.
given to the potential for bulking sludge, higher nutrient discharge and higher aeration needs. Materials and methods
Primary effluent from the Eraser Papers (Nexfor) bleached hardwood Kraft pulp mill in Thurso was deliv ered every two weeks to Ecole Polytechnique, kept at 4°C in a cold room. Eorty litre subsamples were transferred as needed into a slowly mixed tank kept in a refrigerator for pumping(about 17 L/d)into the bioreactor set-up illustrated in Figure 1. Urea and polyphosphates, similar to the fullscale plant, were added as N and P sources respectively, but in excess to ensure that they would not be limiting for the laboratory-scale experiments. Acid addition (0.5N HCl) ensured that the predatory stage pH would not exceed 8.0. A sludge recycle ratio of about 100% was used. The differ ent periods of experimentation (starting September 1,2000) are summarized in Table 1.
Figure 1. Set-up of the lab-scale LSP unit. The influent was fed directly to the predatory stage to reflect activated sludge treatment conditions.
Operational challenges with the lab-scale unit included preventing sludge accumulation in the settler, a condition met by adding a slow speed (1 rpm) sidewall wiper. Wall growth of higher organisms in the bacterial stage needed to be prevented, a concern that is not expected to be signifi cant at full scale due to the much lower wall surface to vol
ume ratio. The addition of about fifteen 4 mm plastic beads
activated sludge process for the predatory stage, Ghyoot and Verstraete (2000) reduced the sludge production even fur ther by another 20% to 30%. A critical review of the poten tial of this process for the pulp and paper industry (Stuart et ai, 1999) highlighted the potential benefits of such a proc ess while indicating that proper consideration should be
viding a hydraulic water break for the influent line. All tubing and the bacterial stage glass beaker (stainless steel
By Yves Comeau\ Britta Petersen\ Paul Stuart^, Michel Perrier^ Sylvie Grafts Claude Asselin^
tested less than one week due to excessive wall growth) was changed and cleaned every two weeks. Grab sampling was used for the influent, the bacterial
36
per litre efficiently prevented the development of slimy and threadlike sludge filaments. Backtlow contamination of the refrigerated influent res ervoir by the bacterial stage biomass was prevented by pro
stage effluent(mixed liquor), and the predatory stage mixed Environmental Science & Engineering, June 2002