Environmental Science & Engineering Magazine (ESEMAG) June 2002

Page 36

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

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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


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