Case Study: Wastewater Recycling at a Connecticut Metalworking and Electroplating Facility

Case Study: Wastewater Recycling at a Connecticut Metalworking and Electroplating Facility

ABSTRACT

Industrial wastewater from drawing, cleaning and electroplating processes were combined for pretreatment in a conventional chemical treatment system, and discharged to a river under an NPDES permit. The treatment system had not performed efficiently, and compliance with effluent limitations had been difficult to achieve. An aggressive water conservation program has been implemented. Plant modifications have been made to separate plating and non-plating wastewater, and to provide separate treatment systems. An advanced filtration system allows plating wastewater to be recycled for process uses within the plant. Reject from this filtration system, as well as pretreated non-plating wastewater, are discharged to the sanitary sewer. The water conservation program and recycling system are beneficial to this facility from regulatory and economic perspectives.

INTRODUCTION

A Connecticut industry discharged treated process wastewater to a river under an NPDES permit. Tube drawing, cleaning and rinsing, and chromium and nickel plating and rinsing wastewaters were combined in holding and equalization tanks. The combined wastewater was subjected to a typical chemical treatment and solids removal system that includes equalization, hexavalent chromium reduction, pH adjustment, coagulation and flocculation, solids separation, sand filtration and final pH adjustment.

The previous chemical pretreatment system was installed in 1999, but the system was labor intensive, and the facility had difficulty achieving discharge permit limitations on a consistent basis. Prior to implementing an aggressive water conservation program, the facility was discharging approximately 60,000 gpd to the river.

The plant adopted a corporate philosophy that encourages efficiency and recycling. This philosophy, and the immediate need to consistently comply with discharge permit limitations, caused the facility to examine alternatives to discharging wastewater under an NPDES permit. Discharging all pretreated process wastewater to the sanitary sewer was considered, however the connection and discharge rate structure of the local POTW made water conservation and recycling the preferred option for wastewater management.

WATER CONSERVATION PROGRAM

Each manufacturing process step (plating and non-plating) was reviewed in detail. This review included the purpose and need for each step, the amount of water used and the type and amount of chemicals used. Where possible, water consumption was reduced by implementing countercurrent rinsing, reducing flow rates, changing some once through rinse overflows to batch dumps, collecting once through non-contact cooling water for use in rinse operations, and eliminating several large NCCW flows by installing chillers. Some large water leaks were fixed, redundant water rinses were eliminated, and rinses that had historically been left running 24 hours per day, 7 days per week were identified. The size, age and complexity of the plant, and the amount of unused piping in pipe trenches, had previously made some of these water-conserving measures less obvious.

Water conservation efforts are complete, and on a plant-wide basis the facility has reduced water consumption by more than 50%. The recycling system is designed to allow approximately 80% of this wastewater to be reused in plating rinse tanks and in cleaning operations.

Implementation of the water conservation program required capital expenditures, plant labor and outside consulting labor. The total cost to design and implement the program was approximately $40,000, with a payback period of approximately one year. This payback period of one year is strong evidence that a water conservation program can be highly beneficial from an economic perspective.

Significantly reducing water use presented some challenges in maintaining compliance with the facility’s NPDES permit, particularly in the case of aquatic toxicity. Compliance was more easily achieved prior to the water conservation program. This is thought to be primarily due to previous dilution of waste streams.

WATER RECYCLING PROGRAM

During design and implementation of the water conservation program, detailed process flow and water balance diagrams for current plant conditions were prepared. All of the facility sumps (12 total) discharge to holding and equalization tanks for similar treatment. Waste streams from some of the tanks and all of the sumps were characterized by sampling and analysis, and the results evaluated with the goal of recycling in mind.

The results of the sampling and analysis program, in addition to process knowledge, suggested that the plant operations, as well as the chemical treatment system, should be split in two parts from a waste management perspective. The majority of drawing and cleaning wastewater flows will be chemically pretreated and discharged to the sanitary sewer. Following treatment in a separate chemical treatment system, plating rinsewater and some low quantities of cleaning wastewater that combine with plating rinses undergo advanced filtration to remove residual organics and total dissolved solids (TDS). Effluent from this advanced filtration system is re-used in initial plating rinses and other less-critical processes throughout the plant.

To simulate the proposed wastewater management conditions, a sampling and analysis program was implemented to enable the design of chemical treatment systems for plating wastewater and for non-plating wastewater. Because production was ongoing, and an NPDES permit was in place, a carefully thought out program was necessary to prepare samples that would best represent plating wastewater that would be chemically treated and filtered for recycling. Aliquot samples were collected from tanks or sumps in proportions that would simulate future conditions. The variability in the characteristics of the five sumps that were considered to be the best candidates for recycling can be seen in Table 1.

Table 1
Characteristics of Aliquots from Wastewater Sumps/Tanks

Sump Receives Wastewater From Parameters of Concern
A Cr Scrubber, Process Tanks Low pH, Ni, Cr, Fe
B Heating Coil Condensate, Process Tanks Low pH, Ni
C Process Tanks High pH, Cr, Ni, Fe, surfactants
D Process Tanks High pH, Cr, Ni, surfactants
E Floor spills, Process Tanks Low pH, Fe, Ni, Fe, grit

Aliquot samples were combined and bench treated chemically in the facility’s laboratory. Bench testing allowed the facility to try different treatment chemistries and protocols that were being contemplated, without compromising effluent quality and without the need to obtain authorization from the Connecticut DEP.

Because some of the waste streams that were being contemplated for reuse would represent relatively high contaminant loading to a TDS removal system, it was decided to limit the use of recycled water to initial plating rinses and non-plating applications, and to continue to use municipal water/deionized water for water quality-critical uses such as plating bath make-up and final plating rinses.

In parallel with chemical treatment bench testing, target concentrations for water proposed for reuse were developed. This was accomplished by obtaining water quality requirements suggested by the facility’s plating line consultant, and by obtaining samples of once-through non-contact cooling water that is currently reused successfully for some plating and cleaning rinses. Comparison of bench test results for chemical treatment and the target concentrations are presented in Table 2.

Table 2
Chemical Treatment Results Compared to Target Concentrations

Parameter Bench Test “Effluent” Concentration Target Concentration for Process Uses
Calcium 0.1 NE
Chromium 0.04 < 0.1
Sodium 494 NE
Iron 0.07 < 0.1
Hardness (CaCO3) 5.0 NE
Magnesium 0.5 < 0.1
Nickel 0.1 < 0.1
Chloride 73 NE
Surfactants 0.1 < 0.1
Sulfate 1,000 NE
TDS 1,700 50

NE = Target Concentration Not Established
NE = Target Concentration Not Established

To achieve the goals of the proposed recycling program, the difference in water quality between the target concentrations and the bench testing results must be accomplished by additional treatment equipment. TDS was the driving constituent in choosing a treatment technology, as it is a critical parameter in plating operations, and is not removed by conventional chemical treatment and solids removal systems. Typically, a dual stage reverse osmosis (RO) system is used to remove TDS. This type of system can provide a very high quality effluent that would be suitable for critical plating uses. However, the decision to use recycled wastewater only for non-critical water uses means that the effluent quality that would be expected from a typical dual-stage RO system is not necessary. In fact, the wastewater under consideration for reuse may cause undesirable fouling of RO membranes without pretreatment.

Chemical treatment bench testing effluent (supernatant from settling containers) was provided to a membrane filtration vendor for testing of various treatment options and collection of permeate samples for laboratory analysis. Prior to membrane filtration, wastewater was treated by granular activated carbon (GAC) for removal of surfactants. The treatment options that were tested, and the characteristics of permeate obtained for each, are presented in Table 3.

Table 3
Permeate Quality Using Filtration Options

Parameter

Concentration Following Treatment Options (mg/l)

Nanofiltration Only

Nanofiltration Followed by RO

RO Only

RO followed by RO

Calcium <0.10 <0.10 <0.10 0.119
Chromium <0.005 <0.005 <0.005 <0.005
Sodium 40.9 0.8 1.8 0.2
Iron 0.012 0.003 0.005 0.006
Hardness (CaCO3) <0.010 <0.010 <0.010 0.297
Magnesium <0.01 <0.01 <0.01 <0.01
Nickel 0.081 0.014 0.024 0.016
Chloride 55 <3.0 <3.0 <3.0
Surfactants <0.05 <0.05 <0.05 <0.05
Sulfate <3.0 5 <3.0 <3.0
TDS 100 <5.0 <5.0 <5.0

 

Based on the data obtained from bench testing using various filtration options, GAC adsorption, nanofiltration, and a single RO unit were chosen to process chemically treated wastewater. GAC followed by nanofiltration adequately pretreats wastewater prior to RO. In bench testing, the RO units became clogged but did not foul. This arrangement will minimize labor and extend the life of the RO membranes.

The system installation provides operational flexibility. Bypass valves and piping allow effluent from various stages of treatment to be directed to chosen areas of the plant for use. For example, water that is used for cleaning rinses in the non-plating portion of the facility does not need to have the quality of RO permeate. Effluent from the GAC columns, or from the nanofiltration membranes, may be directed to this area. These types of operational flexibility reduce municipal water consumption and reduce loads on the RO membranes.

Reject from the nanofiltration and RO units is discharged to the sanitary sewer under a state discharge permit. Because ongoing production and then-current discharge permit conditions made pilot testing impractical, if not impossible, the characteristics of the nanofiltration and RO unit reject were determined by performing mass balance analyses around the units. These analyses indicated that for projected discharge frequencies, reject will comply with discharge permit limits.

Design, installation and permitting of facilities that will allow wastewater recycling were completed in August 2002, and the NPDES discharge was eliminated. The portion of plating wastewater that is not reused is discharged to the sanitary sewer under a state sewer discharge permit.

SUMMARY

Depending on the availability of sanitary sewers and the rate structure of the receiving POTW, process wastewater recycling can be achieved in a manner that is beneficial from a cost-benefit perspective. Any practical recycling program is consistent with the goals and policies of state and federal environmental agencies.

The limitations or practicality of recycling certain waste streams should be taken into consideration. Wastewater that would not be cost-effective to treat for reuse should be left out of the recycling program, as long as alternative means for managing these streams are available. Similarly, if wastewater is not recycled for use in water quality-critical applications, an alternate source of water that is suitable for these applications must be available.

A thorough understanding of process flow and water use should be developed in the early planning stages. After the concept for recycling is developed, a carefully conceived sampling and analysis program should be implemented to define and accurately characterize the wastewater that is proposed to be recycled. This is particularly important in facilities that are in operation during the planning and design process.