Sidestreams in Advanced Wastewater Treatment Plants: Problems and Remedies

For Additional Information:
United States December
Environmental Protection 1985
Agency
For additional information contact:
EPA-OMPC(WH-595)
401 M Street, SW
Washington. DC 20460
(202)382-73697371
EPA-WERL (443)
26 West St. Clair Street
Cincinnati. OH 45268
(5131569-7657
&EPA Sidestreams
in Advanced
Wastewater
Treatment
Plants
Problems
and
Remedies
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Sidestreams in Advanced Wastewater Treatment Plants - Problems and Remedies
Introduction
In wastewater treatment plants, sidestreams constitute the
flows generated within the plant in addition to the plant
influent. Such flows include supernatants, backwash
waters, rinse waters, plant drainage water, and any other
types of liquid streams that are produced in the operation
of the wastewater treatment processes. These sidestreams
are, in most cases, recycled to the head end of the
treatment plant, although in some cases selected
sidestreams are sometimes recycled into secondary
treatment units. The volumes of the sidestreams are
generally small (5 to 10 percent) relative to the influent
flow. Although the volume of the sidestreams is small in
comparison with the forward flow of the treatment facility.
the recycling of the sidestreams to the head end of the
POTW can increase the organic loading by as much as 50
percent with a similar increase in solids loading. The
characteristics of sidestreams vary considerably depending
on the type of treatment process and operation practices at
the facility.
A recent study sponsored by EPA, which included visits to
a number of advanced wastewater treatment (AWT)
facilities, identified sidestreams as one of the potential
causes of performance problems in POTW's with advanced
treatment processes. Based on this evaluation, the major
problems created by the recycling of sidestreams on the
operation and performance of POTW's are identified, and
methods to minimize or eliminate the problems are
discussed.

Types of Sidestreams
The types of sidestreams generated from different AWT
processes vary according to the type of process and its
method of operation. Figure 1 shows a process schematic
of a POTW and the types of sidestreams that may be
generated in the facility.
Backwash wastewater and supernatant/filtrate are the two
major types of sidestreams common in AWT processes.
Figure 1. Process Schematic of
Advanced Wastewater
Treatment (AWT)
Facility.
Tertiary filters and granular activated carbon systems
produce backwash wastewater as a sidestream. The
volume of backwash water may vary between 1 to 5
percent of the amount of wastewater influent to the
AWT unit.
In wastewater treatment facilities with granular activated
carbon (GAC) systems, a carbon regeneration system
utilizing a multiple hearth furnace is often used. As shewn
in Figure 2 this process generates unique sidestreams of its
own. Sidestreams generated from this system include: (1)
the water used to transport the spent carbon to the
regeneration facility, and (2) the water used to quench and
wash the regenerated carbon to remove the carbon fines.
These sidestreams are typically recirculated to the head
end of the wastewater treatment facility.
In addition to the sidestreams generated by AWT
processes, there are a number of non-AWT processes in
POTW's that produce sidestreams. These sidestreams are
also recirculated to the head end of a POTW and ultimately
combine with those sidestreams emerging from the AWT
processes. The combined effect of these sidestream flows
can have significant adverse effects on the operation and
treatment efficiency of the mainstream treatment
processes.

Characteristics of Sidestreams
Very little data on sidestream characteristics are available.
This is mainly because POTW operators typically
concentrate on monitoring mainstream processes and
seldom monitor sidestreams. Based on the limited
information obtained from POTW's and literature review, a
description of the characteristics of the sidestreams
emerging from AWT processes is presented in Table 1. It is
evident that the two major characteristics of sidestreams
are high organic content (BOD) and total suspended solids
(TSS). These sidestreams can exert a high oxygen demand
on mainstream processes. The characteristics of
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sidestreams from non-AWT processes are presented in
Table 2.
The data indicate two salient features. First, there is a wide
range in the concentration of BOD and TSS present in
sidestreams. For example, the BOD of the supernatant
from an anaerobic digester varies from 100 to 2,000 mg/L,
and the TSS ranges between 100 and 10,000 mg/L.
Second, BOD and TSS concentrations in the sidestreams
can be extremely high. Assuming a concentration of 300
mg/L of BOD in the raw wastewater influent to a POTW,
the organic strength of a sidestream can be 5 to 10 times
that of the mainstream. However, the sidestream flow volume
may be considerably small relative to the mainstream flow,
and sidestream flows are generally intermittent. Therefore,
the impact of these sidestreams on AWT process
performance must be determined on a case-by-case basis,
considering these factors.
CARBON SLURRY WATER
Figure 2. Sidestreams Generated in the Carbon
Regeneration System.

Problems Due to Sidestreams
The degree of impact that recycling of sidestreams will
have on AWT processes is determined by the following
factors:
• Characteristics of the sidestreams.
• Volume of sidestream flows.
• Frequency of sidestream flow addition to the
mainstream flow.
As mentioned earlier, it is difficult to obtain sufficient
quantitative information on all of these factors to generalize
the impacts of the sidestreams on the AWT processes.
However, the operators at the plants visited reported
definite cause-and-effect type observations.

Suspended Solids and Fines
One of the common problems reported is the poor settling
characteristics observed in the primary clarifier due to the
presence of fine suspended solids in recirculated
sidestreams. The sidestreams that contribute these
suspended solids are: (1) carbon slurry water containing
carbon fines from the GAG regeneration system, (2)
backwash wastewater from tertiary filters and GAG units,
and (3) filtrate from vacuum filters. The fine suspended
solids are not easily removed in the primary and secondary
treatment processes and are carried over to the AWT
process units. High suspended solids concentration in the
GAG influent can coat the surface of the carbon media.
This reduces the adsorptive capacity of the carbon and
results in poor BOD removal. The frequency of
backwashing is also increased due to the fouling of the
media by the suspended solids. In tertiary filters, high
suspended solids in the influent can clog the media and
result in short filter runs.
In order to minimize the impact of the suspended solids
loading from these sidestreams, flocculants such as alum,
ferric chloride, or organic types of coagulants (polymers)
may be added to primary or secondary clarifiers to capture
the fine suspended solids for improved settling (Figure 3).
If the sidestream flow is significant relative to the plant
influent flow and creates major problems in clarification,
pretreatment of the individual sidestreams might be
considered. As shown in Figure 3, the flow from the
backwash wastewaters and the carbon slurry water can be
equalized and the suspended solids removed by settling
before the flow is recycled. Flocculant may be added to
improve settling of the solids.

Hydraulic Surging
Another impact of the recycling of sidestreams is the
hydraulic surge induced on the mainstream flow. This
occurs when the backwash wastewaters from the tertiary
filters and GAG units are recycled to the head end of the
plant. As a result of the hydraulic surge, suspended solids
removal in the primary clarifier is adversely affected. A
cascading effect follows and results in high suspended
solids loading on the subsequent secondary and AWT
processes.

The impact of the hydraulic surge can be minimized by
equalizing the sidestream flows (Figure 3) and controlling
the recyle to the mainstream flow using a flow pacer. It is
suggested that the equalized sidestream flow be recycled
during periods of low influent flow to the plant.

Organic Overloading
Recycling of sidestreams can create significant problems of
overloading of treatment processes due to high
concentration of organic matter in the sidestreams. This
problem arises primarily from sidestreams generated by
non-AWT processes (Figure 1). These sidestreams often
cause intermittent organic peak loads. It is suggested that
these sidestreams be equalized and aerated before being
recycled into the head end of a treatment facility to
minimize the oxygen demand exerted on the
mainstream flow. One way to accomplish this preaeration
is to pass the recycled sidestream through an aerated grit
chamber if one exists.
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In biological phosphorus removal processes, (PhoStrip,
A/O, and Bardenpho) a fraction of the phosphorus
enriched sludge is often thickened in a gravity thickener
and the supernatant (which is a sidestream) is returned to
the head end of the treatment plant. In the sludge
thickener, under anaerobic conditions, part of the
phosphorus in the sludge is resolubilized and returned to
the primary clarifier, along with the thickener supernatant.
As indicated in Figure 3, this problem can be minimized by
either of the following measures: (1) addition of dilution
water to the thickener to prevent the sludge from becoming
anaerobic or (2) use of the dissolved air flotation process
for sludge thickening.
Figure 3. Suggested
Modifications in AWT
Facility to Minimize
Sidestream Problems.
Summary
The performance evaluation of sidestreams in AWT plants
indicates that sidestream flows, although small in volume
relative to the mainstream flow, can adversely affect the
treatment processes. The significant problems due to the
recycling of the sidestreams are: (1) poor settling in
clarifiers due to fine suspended solids, (2) shock loads of
high organic matter & TSS, and (3) hydraulic overloading
due to intermittent flow surges. Possible remedial measures
to consider include the following: (1) addition of flocculant
to the sidestream flow to improve the settling
characteristics, (2) sidestream flow equalization, and (3)
aeration of sidestreams containing high concentrations
of BOD to minimize the oxygen demand on the
mainstream flow.
Table 1. Description of Sidestreams From AWT Process Units
Treatment Process
• Tertiary Filtration

* Granular Activated
Carbon

• Phosphorus Removal
Processes
(PhoStrip, A/O,
Barden Pho)
• Ion Exchange

• Biological
Nitrification
• Damnification
• Independent Phy-Chem.

• Fluid Bed
• Microstraining
Type of Sidestreams
Backwash Wastewater

Backwash Wastewater
Carbon Slurry
Water (Regeneration)
Chemical Sludge
Biological Sludge

Regeneration
Wastewater
Biological Sludge

Biological Sludge
Chemical Sludge

Backwash Wastewater
Backwash Wastewater
Characteristics
High TSS (100-1000 mg/L), High BOD
(100-1300 mg/L)
High TSS; High BOD
High TSS

High Phosphorus and Lime

High TSS; High BOD, Low pH

High TSS; High BOD

High TSS; High BOD
High TSS; High BOD, High Concentration
of Lime, Alum, Ferric Chloride Etc.
High TSS; High BOD
High TSS; High BOD
Table 2. Description of Sidestreams From Non-AWT
Process Units
Characteristics
Treatment Process
Gravity Thickener
Dissolved Air Flotation
Anaerobic Digestion
Aerobic Digestion
Heat Treatment
Wet Air Oxidation
Puritax
Sludge Lagoon
Sludge Drying Bed
Vacuum Filter
Belt Filter Press
Filter Press
Centrifuge
Type of Sidestream
Supernatant
Supernatant
Supernatant
Decant
Decant Liquor
Decant Liquor
Decant Liquor
Supernatant
Underdrainage
Filtrate
Filtrate
Filtrate
Centrate
BOD (mg/L)
100-
50
100-
100-
1,600 -
3,000 -
1,000-
100-
20-
100-
50-
50 .
100-
1,200
1,200
2,000
2,000
12,000
15.000
3,000
200
500
1,000
500
250
2,000
TSS (mg/L)
200
1 00
DO
100
100
100
100
50
20
200
100
50
200
- 2.500
- 2,500
-10,000
-10,000
- 10,000
- 10,000
- 3,000
- 200
- 500
- 4,000
- 2.000
- 1,000
- 20,000
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