Increasing Wastewater Treatment Plant
Capacity and Efficiency
By Chemical Addition
C. MICHAEL ROBSON, Manager
GARY L. NICKERSON, Project Engineer
JEFFREY L. VAN ATTEN, Project Engineer
Engineering-Science, Inc.
Washington, D.C.
INTRODUCTION
The County of Fairfax, Virginia presently operates four wastewater treatment plants
which discharge directly to the Potomac River or immediate estuarian streams (Figure 1).
Three of these plants, Westgate, Little Hunting Creek, and Dogue Creek, were much
criticized by conservation groups and other interested citizens in early 1970 for providing
inadequate treatment and thus contributing to the pollution of the Potomac River. In June
1970, Fairfax County submitted a detailed report to the Virginia State Water Control
Board (VSWCB) which outlined interim improvements to increase the treatment capacities
of the three plants (1). Further, a long range program was initiated at this time which
provided for terminating operations at Little Hunting and Dogue Creek plants by May 1,
1972, and at the Westgate plant by July 1, 1973. A large part of the interim improvement
program relied on the incorporation of chemical treatment.
This paper describes the methods by which Fairfax was able to significantly increase
the treatment capacity and efficiency of the Westgate, Little Hunting Creek and Dogue
Creek wastewater treatment plants. The description is divided into three main parts: 1)
modifications and results achieved at the Westgate Plant that provided primary level
wastewater treatment; 2) modifications and results at the two County biofiltration plants,
Little Hunting Creek and Dogue Creek; and 3) sludge handling problems which
accompanied the chemical addition modifications.
WESTGATE WASTEWATER TREATMENT PLANT
A major portion of Fairfax County's pollution abatement program was concerned
with increasing the capacity of the Westgate Wastewater Treatment Plant, which was
constructed in 1954 to provide an intermediate level of treatment for 8 million gallons per
day(MGD) of sewage. The original plant (Figure 2), a pioneering effort on the Potomac for
its day, was designed to achieve treatment efficiencies in excess of 50 percent Biochemical
Oxygen Demand (BOD5) removal. Liquid stream treatment was accomplished in two bays
of a single rectangular tank, baffled into three zones: primary settling, aeration and final
settling (Figure 3). Sludge digestion, dewatering, drying and pelletizing equipment was
installed as part of the original construction, but only the sludge dewatering equipment has
been used for the majority of the operational life of the plant. A sludge pumping station with
two 25,000 gallon sludge decanting chambers was later constructed for use in conjunction
with the two vacuum filters to dewater raw primary sludge. The dewatered sludge cake was
then hauled to a landfill in 10 cubic yard "load-lugger" containers. A description of the main
treatment components of the original plant is given in Table I. The average daily flow at
Westgate, by early 1970, had increased to 12 MGD, and BOD5 removals, at times, dropped
as low as 35 percent.
A crash program of laboratory and plant scale testing was conducted in June and July
1970 to determine the effectiveness of chemical treatment at the Westgate facility. The
results of the subsequent plant scale trial, initiated in July using temporary facilities, were
sufficient to cause the County officials to authorize continued chemical addition by using
temporary facilities until the permanent equipment could be installed (2).
95