This study evaluated the potential impacts of recycling filter backwash water (FBWW) on coagulation chemistry. Three bench-scale processes were tested: no recycle, recycle of untreated FBWW, and recycle of alum-treated FBWW. Preliminary results showed enhanced removal of organic materials like total organic carbon, dissolved organic carbon, UV-254, and turbidity as the percentage of FBWW recycle increased. Zeta potentials at constant coagulant dosages indicated unique trends for each source water, suggesting removal mechanisms beyond just charge neutralization. Improved flocculation and sedimentation were also observed, possibly due to increased bonding sites from introduced FBWW during coagulation. Further analysis of specific water quality parameters and

1. Evaluation of Potential Impacts of Recycling
Filter Backwash Water on Coagulation Chemistry
A. Gottfried1 and M. Walsh2
1Department of Civil Engineering, McMaster University, Hamilton, ON
2Department of Civil and Resource Engineering, Dalhousie University, Halifax, NS
RATIONALE The recovery and reuse of filter backwash water (FBWW) is viewed as a possible path towards minimizing waste streams and optimizing treatment
performance in a conventional drinking water treatment plant. Recycling FBWW can utilize the existing waste stream to advance residual management practices
and treatment options for water utilities that are required to meet more stringent discharge quality requirements.
INTRODUCTION
FBWW consists of metals, colloidal material, natural organic matter and
microorganisms that concentrate in the filter media during the filtration
process. In Canada, recycling FBWW is not a common practice due to
concerns over potential treatment upsets and impacts to finished water
quality. However, many utilities in the United States recycle FBWW
streams to the head of the plant as a management strategy. The objective
of this research is to examine the changes in coagulation chemistry that can
occur with FBWW recycle by measuring specific water quality parameters.
EXPERIMENTAL DESIGN
Three process designs were evaluated at bench-scale:
Process 1: No recycle → to determine typical treated water quality
Process 2: Recycle of untreated FBWW → impact of raw residuals on water quality
Process 3: Recycle of alum treated FBWW → impact of treated residuals on water quality
Parameters Measured
Bulk Water Quality Metal Analysis Coagulation Behaviour
- TOC/DOC, pH - Manganese - Zeta Potential
- UV-254, Turbidity, Colour - Aluminum
Water Sources:
WTP 1: Windsor, NS: (WTP1) High TOC, conventional FBWW
WTP 2: Hantsport, NS: (WTP2) Low TOC, membrane concentrate
CONCLUSIONS
Preliminary results have demonstrated that there may be potential benefits to recycling FBWW
streams within a drinking water treatment plant in terms of enhanced removal of organic material:
• Bench results showed enhanced removal trends for TOC, DOC, UV-254 & turbidity as the % recycle increased.
• Zeta potentials at constant coagulant dosages showed unique trends for each source water, indicating the
presence of coagulation mechanisms other than charge neutralization for the removal of NOM and turbidity.
• Improved flocculation and sedimentation removal rates observed may be due to the increased number of
bonding sites available due to introduction of FBWW during coagulation.
Further Information
Contact: Dr. Margaret Walsh
E-mail: mwalsh2@dal.ca
Acknowledgements
Natural Sciences and Research Council of Canada
Municipality of Hantsport (Jeff Lawrence & Don Silver)
Municipality of Windsor (Todd Richardson)
RESULTS: Process 2 - WTP 1
Flocculation &
SedimentationRapid Mixing
Dual Media Filtration
Process 2 & 3
FBWW Recycle
Filtrate to Clearwell Backwash Water
Process 3 FBWW Alum Addition
Raw Water
Alum Addition
Raw Water
Solids To Sludge
Thickeners
RESULTS: Process 1 - WTP 2
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0 10 20 30 40 50
Alum Dose (mg/L)
UV-254
RW2 UV-254
-25
-20
-15
-10
-5
0
5
10
0 10 20 30 40 50
Alum Dose (mg/L)
ZetaPotential(mV)
RW2 UV-254
-12
-10
-8
-6
-4
-2
0
0 2 4 6 8 10
Recycle %
RW1 Alum Dose = 30mg/L
ZetaPotential(mV)
RW1 Zeta Potential
0.00
0.02
0.04
0.06
0.08
0 2 4 6 8 10
Recycle %
RW1 Alum Dose = 30mg/L
UV-254
2.60
2.65
2.70
2.75
2.80
2.85
2.90
TOC(mg/L))
RW1 UV-254 RW1 TOC