Wastewater Treatment Technology and Applications in Industrial Facilities

Wastewater Treatment Technology and Applications in
Industrial Facilities
Treatment that industrial facilities give wastewater before discharging it to the local
wastewater treatment facility is referred to as "pretreatment." Methods for providing
pretreatment can be divided into four categories: physical, chemical, biological, and
membrane. This briefing addresses the first three categories; membrane technology is
addressed in a separate End-Use Briefing. A pretreatment system may use one or several
pretreatment processes.
In general, physical pretreatment requires the least energy but is least effective in
reducing pollutant concentrations. Biological processes tend to have the greatest energy
cost, but are highly effective in reducing conventional pollutants. Chemical processes
tend to lie in between the other two both in energy use and effectiveness.
This end-use briefing provides a quick tutorial of wastewater treatment technology and
applications in industrial facilities. The following topics are addressed:
•
Conceptual overview of industrial wastewater treatment.
•
Industrial pretreatment processes.
•
Typical pretreatment processes for selected industries.
•
Items to look for in the field for efficient operations.
•
A list of references to other documents that provide more detailed information on
the topics in this briefing.
Overview
Most industrial facilities discharge their wastewater to a local treatment facility. Due to
Federal regulations regarding pretreatment and fees charged by local wastewater
treatment facilities, many industrial facilities provide pretreatment. Federal regulations
prohibit the discharge of wastes incompatible with: 1) the conveyance of the wastewater
to the local facility (such as highly odorous compounds), 2) the processes at the local
facility (such as chemicals that would inhibit biological treatment) or 3) the use or
disposal of the treated wastewater or resulting sludge (such as certain pesticides). The
EPA has set pretreatment standards which apply to more than 40 specific industry
categories. Pretreatment standards also apply to the discharge of more than 120 specific
pollutants. Details of pretreatment requirements are in 40 CFR Part 403 "General
Pretreatment Regulations for Existing and New Sources of Pollution".
Figure 1: Schematic of an Activated Sludge Wastewater Treatment
Facility
Most treatment facilities charge industrial users a fee based on the amount and types of
pollutants discharged. Other considerations that affect the amount of pretreatment a
particular industrial facility would provide include:
•
Pollutant or pollutants to be removed.
•
Site constraints, such as space availability for equipment.
•
Amount and variation of wastewater flow and pollutant concentration, both hourto-hour basis and seasonal
A schematic of a typical activated-sludge, secondary wastewater treatment facility is in
Figure 1. The energy used by any unit process can very dramatically from one treatment
facility to another. Variation in energy use can be a result of differences in flow, pollutant
loading, process control methods or prior or subsequent unit processes used at different
facilities. Typical electric energy use in such a facility could be distributed as follows:
Influent, intermediate, or effluent
pumping
Primary sedimentation
Activated sludge
Sludge processing
Lighting, monitoring, controls
Disinfection (by purchased
chemicals)
Odor control
10-20%
2-5%
30-70%
10-50%
1-3%
1-3%
1-2%
(The related topics of sludge processing and hazardous waste treatment are not discussed
in this End-Use Briefing.)
Industrial Pretreatment Processes
Following is a brief description of common physical, chemical and biological pretreatment processes:
Physical
•
Screening is removal of coarse solids by use of a straining device.
•
Sedimentation is gravity settling of pollutants out of the wastewater.
•
Flotation is the use of small gas bubbles injected into the wastewater which
causes pollutant particles in the wastewater to rise to the surface for subsequent
removal.
•
Air stripping is removal of volatile and semi-volatile organic compounds from
wastewater by use of air flow.
Chemical
•
Neutralization is adjustment of alkalinity and acidity to the same concentration
(pH 7).
•
Precipitation (ppt) is addition of chemicals to wastewater to change the chemical
composition of pollutants so that the newly formed compounds settle out during
sedimentation.
•
Coagulation is use of chemicals to cause pollutants to agglomerate and
subsequently settle out during sedimentation.
•
Adsorption is use of a chemical which causes certain pollutants to adhere to the
surface of that chemical.
•
Disinfection is use of a chemical (or other method such as ultraviolet radiation) to
selectively destroy disease-causing organisms. (Sterilization is the destruction of
all organisms.)
•
Breakpoint chlorination is the addition of chlorine to the level that chloramines
will be oxidized to nitrous oxide and nitrogen, and chlorine will be reduced to
chloride ions.
Biological
•
Air activated sludge is an aerobic process in which bacteria consume organic
matter, nitrogen and oxygen from the wastewater and grow new bacteria. The
bacteria are suspended in the aeration tank by the mixing action of the air blown
into the wastewater. This is shown schematically in Figure 1. There are many
derivations of the activated sludge process, several of which are described in this
section.
•
High purity oxygen activated sludge is an aerobic process very similar to air
activated sludge except that pure oxygen rather than air is injected into the
wastewater.
•
Aerated pond/lagoon is an aerobic process very similar to air activated sludge.
Mechanical aerators are generally used to either inject air into the wastewater or
to cause violent agitation of the wastewater and air in order to achieve oxygen
transfer to the wastewater. As in air activated sludge, the bacteria grow while
suspended in the wastewater.
•
Trickling filter is a fixed film aerobic process. A tank containing media with a
high surface to volume ratio is constructed. Wastewater is discharged at the top of
the tank and percolates (trickles) down the media. Bacteria grow on the media
utilizing organic matter and nitrogen from the wastewater.
•
Rotating biological contactor (RBC) is a fixed film aerobic process similar to the
trickling filter process except that the media is supported horizontally across a
tank of wastewater. The media upon which the bacteria grow is continuously
rotated so that it is alternately in the wastewater and the air.
•
Oxidation ditch is an aerobic process similar to the activated sludge process.
Physically, however, an oxidation ditch is ring-shaped and is equipped with
mechanical aeration devices.
Pollutant
Bio-Chemical Oxygen Demand (BOD)
Total Suspended Solids (TSS)
Nitrogen
Phosphorus
Heavy metals
Fats, Oil and Grease (FOG)
Volatile Organic Compounds
Pathogens
Pretreatment Processes
Activated Sludge
Trickling filter or RBC
Aerated lagoon
Oxidation ditch
Sedimentation
Screening
Flotation
Chemical precipitation
Nitrification/denitrification
Air stripping
Breakpoint chlorination
Chemical precipitation
Biological treatment
Air stripping
Biological treatment
TChemical precipitation
Evaporation
Membrane process
Coagulation
Flotation
Biological treatment
Membrane process
Air stripping
Biological treatment
Carbon adsorption
Chemical disinfection
UV radiation
Table 1: Selected Pollutants and Associated Pretreatment Processes
Table 1 identifies the pretreatment processes most commonly used to treat specific
industrial pollutants. Pretreatment often involves more than one process and the order of
multiple processes is very important. With wastewater whose pH is high, neutralization
would be needed prior to using a biological process, as high pH would adversely affect
the growth of the organisms. The order of the treatment processes also affects operating
costs. Primary sedimentation often precedes biological treatment, for several reasons: to
remove some of the BOD by a low operational cost method; to remove waste matter that
may adversely affect the biological process; and to provide some flow equalization prior
to the wastewater entering the biological treatment process.
Typical Pretreatment Processes for Selected Industries
Several industries, their associated wastewater pollutants and common pretreatment
processes used to treat those pollutants are shown in Table 2.
Industry
Associated
Wastewater
Polluants
Pretreatment Process
Apparel
Textiles
BOD, TSS,
alkalinity
BOD, TSS,
Chromium
Alkalinity, BOD,
turbidity
Neutralization, chemical precipitaion,
biological treatment
Sedimentation, biological treatment
BOD, suponified
soaps
Floatation and skimming, chemical ppt
Brewed beverages
Meat & poultry
BOD
BOD
Rice
Bakeries
BOD, TSS
BOD, FOG,
detergents
BOD, TSS,
alkilinity
Centrifugation, biological treatment
Screening, sedimentation, biological
treatment
Chemical precipitation
Biological treatment
Leather goods
Laundry
Screening, chemical precipitaion,
adsorption
Detergents
Food
Soft drinks
Neutralization, screening, biological
treatment
Pharmaceuticals
BOD
Evaporation, drying
High or low pH,
TSS, inorganic
compounds
Sedimentation, neutralization, biological
treatment
Acidity, heavy
metals
Neutralization, sedimentation, chemical
precipitation
Hign or low pH,
volatile organic
compounds
Neutralization, biological treatment
Pulp and paper
Metal-plating
Plastics and
resin
Table 2: Selected Industries, Associated Wastewater Pollutants, and Pretreatment
Processes
What to Look for in the Field for Efficient Operation
Energy demand varies significantly between facilities, depending on any of several
parameters, including: variation in wastewater flow rate, pollutants removed, treatment
processes utilized, physical characteristics of the pretreatment site, methods of process
control, processes used for sludge treatment and amount and type of air emissions
controls. For the above reasons it is difficult to provide accurate energy use data for a
particular unit process. However, there are operational aspects of a unit process which
relate to its efficiency that are relatively independent of the particular site characteristics.
Table 3 suggests things to look for in the field to assess the efficiency of operation of
various pretreatment processes.
Pretreatment
Process
Physical
Screening
Sedimentation
Centrifugation
Air stripping
Items to Look for in the Field for Efficient
Operation
No blinding or clogging of screens, no excessive build-up
of material on the screen
Low flow rate, no short circuiting of flow, no floating
sludge, scum removal if appropriate
Knowledgeable operations staff
No scaling of packing and piping, or freezing problems at
low temperatures
Chemical
Neutralization
Precipitation
Coagulation
Adsorption
Disinfection
pH monitoring, automated chemical feed, adequete mixing
Automated chemical feed system, adequate mixing &
contact timer
Automated chemical feed system, adequate mixing &
contact timer
Efficient means of regeneration is key to preformance
Automated chemical feed system, adequate mixing &
contact timer
Biological
Activated sludge
Trickling filter
Rotating biological
contactor (RBC)
Fine bubble aeration, even distribution of air and mixing,
dissolved oxygen concentration monitoring, air flow turndown capability, no bulking/floating sludge
Method for positive air circulation, even & periodic
dousing of filter media
Steady shaft rotation
Table 3: Pretreatment Processes and Items to Look for in the Field for Efficient Operation
Tips for Efficient Operation and Maintenance
The areas of largest energy use in wastewater treatment facilities are usually pumping the
wastewater, providing biological treatment and solids processing.
Pumping
•
Implement process control improvements to optimize flow rates, for example:
minimize trickling filter recirculation and under-pumping sludge from clarifiers.
•
Consider using variable frequency drives when flow rates are variable.
Biological Treatment
•
Optimize primary treatment efficiency. BOD removed by primary treatment
requires less energy than BOD removed by biological treatment, also surplus
sludge should not accumulate in the primary clarifier to avoid dissolving and
carry-over to the secondary process.
•
Control amount of aeration to avoid excessive dissolved oxygen.
•
Improve aeration system efficiency. For example, fine-bubble diffusers can
increase oxygen transfer efficiency in comparison to coarse-bubble diffusers.
More Information
Call 1-800-468-4743 for more information about PG&E's energy efficiency programs
and other services.
References
1. Alberi, et al, "Pretreatment of Industrial Wastes," Manual of Practice No. FD-3,
Water Environment Federation, 1994.
2. Edwards, "Industrial Wastewater Treatment," Lewis Publishers, 1995.
3. Metcalf, Eddy, "Wastewater Engineering," Third Edition, McGraw-Hill, Inc.,
1991.
4. Nemerow, "Liquid Waste of Industry," Addison-Wesley Publishing Co., 1971.
5. Miorin, et al, "Wastewater Treatment Plant Design," WPCF Manual of Practice
No. 8, Water Environment Federation, Second Printing, 1982.
6. Renzo (editor), "Pollution Control Technology for Industrial Wastewater," Noyes
Data Corporation, 1981.