By Peter McCarthy,M.A.Sc. and Zhiping Qiu, M.Sc.E., P.Eng.
Sludge thickener (foreground) and SBR basins (background).
In 2005, ADI Systems Inc. of
Fredericton, New Brunswick,
completed commissioning for a
large ADI-SBR wastewater treatment
system for Guangzhou Paper,
one of China's largest pulp and paper
mills. In North America, ADI Systems
normally provides turnkey installations,
but in China we typically supply
a technology package, which for this
project included: process design;
detailed design of selected components;
design manual (to guide local
detailed designers); ADI-ASD
decanters; jet aeration system; instrumentation;
control valves; DCS control
system; operator training; and
commissioning.
The paper mill is located in
Guangzhou city centre on the shore of
the Pearl River in southern China. The
mill has chemi-thermomechanical pulping,
sulfite pulping, groundwood pulping,
and paper-making operations.
Before installation of the Sequencing
Batch Reactor (SBR) system, approximately
5000 m3/d of wastewater was
pretreated in a high-rate aerobic system
and low-strength wastewater was pretreated
in two dissolved air flotation
units.
New effluent regulations required
the mill to install a secondary wastewater
treatment system. The new discharge
limits for COD, BOD, and TSS
are 200 mg/l, 30 mg/l, and 200 mg/l,
respectively.
The design wastewater characteristics
for the biological treatment plant
are summarized in Table 1.
ADI’s partners on this project
included the mill, which provided project
and construction management, and
the Changsha Design Institute that
provided detail design.
The new wastewater treatment system
includes the following major components:
influent equalization tank (6000 m3);
influent pumping system;
nutrient addition systems for nitrogen
and phosphorus;
five SBRs (14,000 m3, each); and
gravity WAS thickener.
The ADI-SBR system is comprised
of five on-grade, common wall, castin-
place concrete basins. Each SBR
tank is 31.5 m by 68 m and has a 7.5 m
wall. The treatment plant was constructed
using 12,000 tonnes of concrete
and 1600 piles. The average
depth of piles is 14 m with a pile diameter
of 40 cm. The total weight of reinforcing
steel used in the project was
1400 tonnes.
During the construction period, the
concrete and steel prices increased
substantially, and the contractor had to
resolve the price escalation problems
with the mill before resuming the construction
work. Therefore, there was a
delay in the treatment plant construction
schedule. However, pressure from
the local government was so high that
the mayor of Guangzhou promised the
public that he would swim in the Pearl
River in 2005!
Nutrient supplementation was
required for biological treatment. Urea
(dry) was selected for nitrogen, and
75% phosphoric acid was selected for
phosphate. Urea was purchased in
bags and dissolved and stored in fiberglass
tanks. Phosphoric acid was transferred
into the fiberglass storage tanks
directly from tanker trucks. Each storage
tank is equipped with a level-indicating
transmitter. The daily chemical
consumption is calculated based upon
the liquid level change.
Each basin is equipped with a simple
influent distribution system, waste
sludge collection manifold, and jet aeration
system, all of which are located
on the basin floor. Each basin is also
equipped with three ADI-ASD
decanters and instrumentation. The
instrumentation includes a pH/temperature
sensor and analyzer, a dissolved
oxygen sensor and analyzer, a level
indicating transmitter, and a level
switch.
Each SBR basin is equipped with a
jet aeration system, with two jet aerators
running the length of each basin
for oxygen supply and mixing. The jet
aeration system has demonstrated that
it has excellent efficiency in transferring
oxygen from the air to the process
water and is quite reliable in terms of
maintenance. Each basin is connected
to one dedicated, multi-stage, centrifugal
blower. Energy consumption is
minimized by a control valve on the
suction side of the blower. As oxygen demand increases or decreases, the
blower power draw and air flow is
increased or decreased via the inlet
control valve.
The control system is designed for
flexible operation of the system. The
DCS operator interface is intuitive and
bilingual. The SBR cycle length is
variable, and the system can be run
with any number of basins on line.
Individual event durations (Fill,
Fill/Aerate, Aerate, Mix, Settle, and
Decant) are also adjustable. The system
can be operated using continuous
flow to the SBR system, holding the
Equalization Basin (EQB) level constant,
or flow can be stored in the EQB
for shorter Fill durations. The aeration
system can also be operated manually
should the dissolved oxygen monitoring
system be out of service for maintenance.
All setpoints are adjustable by
the operator or supervisory staff.
Two of ADI's process engineers
were on-site for operator training and
commissioning which got underway
late in 2004. One ADI process engineer
spoke Mandarin; this helped with
communications between ADI and the
mill staff. Although the Process
Operations Manual and other start-up
materials were written in English, all
screens on the interface with the DCS
were bilingual (Chinese and English).
During the first six months, the
influent flow had been close to design,
averaging approximately 60,000 m3/d;
however, the influent COD has consistently
been higher than design (of 750
mg/l), frequently averaging over 1,500
mg/l.
Results from the first six months
indicate that the system is performing
very well, although the SBR influent
COD and TSS are constantly above
design conditions (the weekly average
influent COD is frequently more than
double the design value). The average
effluent BOD for the past six months
was 10 mg/l; however, the effluent
COD was higher than discharge limit
(due to the high influent values and the
non-biodegradable COD fraction typical
of pulping wastewaters). The owner
has been very satisfied with the SBR
system performance due to the excellent
BOD removal, low maintenance,
low operating costs, ease of operation,
and its reliability. They are especially
pleased with the system's capability to
handle higher than design COD, BOD,
and TSS concentrations.
Peter McCarthy, M.A.Sc. and
Zhiping Qiu, M.Sc.E, P. Eng.
are with ADI Systems Inc.
Contact: pjm@adi.ca or zpq@adi.ca
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