A company manufacturing bar soap products was founded in 1972 in Quebec, Canada. The business later relocated to Winooski, Vermont, and evolved over the years to become an industry leader in bar soap innovation and sustainability. The company entered the premium natural skincare space in the early 2010s and rebranded. It currently manufactures on a contract basis both bar soaps and liquid skincare products for over 140 name brands worldwide, with 240 employees at its facilities in Winooski (bar soap) and Essex, Vermont (liquid skincare products).
Figure 1. Pre-project influent BOD variability (pretreatment)
As part of its expansion into the liquid skincare product market, the manufacturer began construction of a liquid soap manufacturing line in warehouse space it purchased in Essex in 2014. A review of wastewater discharge permitting requirements for the town of Essex revealed that wastewater strength in terms of biochemical oxygen demand (BOD) would not be a significant issue. However, a high concentration of fats oils, and grease (FOG) was a major concern. Essex found, from prior experience with industrial dischargers releasing FOG into its municipal wastewater collection system, that FOG results in significant operation and maintenance costs for the town, primarily related to sewer blockages. Their sewer ordinance now requires that wastewater generated by connected users not exceed 100 mg/L for FOG.
PROCESS SELECTION
Figure 2. Pre-project influent FOG variability (pretreatment)
Wastewater from the manufacturer’s Essex facility consists primarily of washdown of production and packaging equipment between production runs. Therefore, relatively little wastewater, on the order of 2,000 gal/d (7,570 L/d)m, is generated. Effluent quality analyses obtained prior to design of the pretreatment system showed BOD to average 3,900 mg/L and FOG to average 500 mg/L, with an average pH of about 7.8. While these averages are not out of line for wastewater of this type, the wastewater quality varied greatly, depending upon the product being manufactured at the time.
Since the company contract-manufactures liquid soap, sunscreen, and other skincare products at the Essex location, wastewater quality varies drastically depending on the products made and the size of each batch, which also varies greatly. Figures 1 and 2 show the variability in the wastewater quality between February 2015 and May 2016, before the pretreatment system was commissioned.
Wastewater from the Essex facility was discharged by gravity from the building to a small duplex submersible pump station, which lifted the wastewater to the municipal wastewater collection system. Because of the wastewater’s heavy FOG loading prior to implementing a pretreatment system, the manufacturer had to periodically pressure-wash this pump station with a vacuum tanker to remove accumulated grease, which had in the past caused the pump station to fail.
Figure 3. Liquid skincare products production schematic
Figure 3 depicts the manufacturing process at the Essex facility and the areas that generate wastewater. At this facility, ingredients for specific soap and skincare products are batched in a heated, jacketed kettle. Once the on-site laboratory has confirmed the product quality and ensured it meets the customer’s specifications, it transfers the kettle contents in batches to a tote or mobile kettle where the product can be maintained at a certain temperature while being delivered to the product packaging line for placement into final packaging.
From the product packaging line, individual product units are removed and bundled for packaging and shipping. For each process where the product contacts equipment, a clean-in-place (CIP) system washes and sanitizes the process equipment. Spent CIP system wash water is discharged to a trench drain that passes through the facility. This is the wastewater, containing soap and sunscreen residuals, that is sent to the municipal wastewater collection system as described above.
1. Air entrainment system
Based on its experience, the manufacturer anticipated the new liquid skincare production facility would continue to generate much higher concentrations of FOG than the local limit of 100 mg/L. It found that meeting this limit was not achievable using best management practices on the manufacturing line. Therefore, a pretreatment system had to be designed and constructed that would consistently remove FOG to below 100 mg/L prior to discharge to the town’s wastewater collection system.
Domestic wastewater from this facility is discharged to the same effluent pump station, but was deemed not to contribute to the FOG issue. The town indicated that, while the manufacturer’s average effluent BOD was much higher than in domestic wastewater, this did not pose a problem for the treatment system. It said it could be addressed simply by having the manufacturer pay a high-strength surcharge to cover the additional treatment costs related to the higher organic load. FOG, however, could not be discharged above the permitted limit of 100 mg/L.
In 2015, the manufacturer retained a wastewater engineering consultant to identify and design a treatment process for removal of FOG to both satisfy local wastewater discharge requirements and reduce O&M costs in managing its wastewater. A dissolved air flotation (DAF) wastewater pretreatment system was recommended. For this project, “process wastewater” was defined as equipment and process area washdown, CIP system discharge, and controlled releases of rejected product. No domestic wastewater was included in the pretreatment system influent load.
PROCESS DESCRIPTION
2. Wastewater storage tanks
Since FOG was the target constituent for removal and tends to have a lower specific gravity than water, causing it to float to the surface, DAF was the most efficient means of removal. DAF is a wastewater treatment process that can clarify wastewater by removing suspended matter such as oils or suspended solids. It does so by dissolving air in the water or wastewater under pressure and then releasing the air at atmospheric pressure in a flotation tank. The released air forms tiny bubbles that adhere to the suspended matter, causing the suspended matter to float to the surface of the water where it may be removed by a mechanical skimming device. For this manufacturer, the feed water to the DAF tank is typically dosed with a coagulant (such as ferric chloride or aluminum sulfate) to coagulate the colloidal particles and give them more surface area for the air bubbles to cling to. Flocculants (polymers) also help remove coagulated particles to improve process efficiency.
3. Basket strainers
Manufacturers use different methods of entraining air into the wastewater. Traditionally, a portion of the clarified effluent water leaving the DAF tank is pumped into a small pressure vessel (called the air drum) into which compressed air is also introduced. This results in saturating the pressurized effluent water with air. The air entrainment system used by this manufacturer is based on equipment that uses air induction via a venturi located on the downstream side of a recirculation pump to saturate recirculated wastewater with air. Air flow is controlled with a rotameter and needle valve for system optimization. As the wastewater is pumped through the venturi, the cone-shaped throat constriction increases the fluid velocity, dropping its pressure and producing a partial vacuum. This partial vacuum pulls ambient air into the wastewater stream, and as the wastewater leaves the constriction, its pressure increases back to ambient. This results in the same air saturation effect, but it occurs more efficiently (Photo 1).
4. pH adjustment skid
Because of the facility’s wide variability in wastewater quality and the small volume of wastewater discharge, the design is based on batch treatment. This allows for better process control and provides a consistent wastewater quality entering the pretreatment system to enhance system effectiveness. To achieve this, the design includes two 10,000 gal (37,854 L) polyethylene wastewater storage tanks, sized to allow for several days of wastewater storage, and a pH adjustment system (Photo 2).
Process wastewater collected throughout the building is pumped through fine 0.1 in. (2.5 mm) basket strainers (Photo 3) before entering the storage tanks. These strainers remove plastics and debris that could adversely affect the pretreatment equipment and its performance.
Once enough wastewater accumulates to completely fill one of the storage tanks, this tank is hydraulically isolated, a mechanical mixer homogenizes the tank contents, and wastewater is recirculated through the tank using a centrifugal pump. A 2 hp (1.49 kw) mechanical mixer is suspended on a structural steel platform above each tank. This platform, shown in Photo 2, allows for mechanical mixing, which is more efficient and less costly than pumped mixing. The platform is designed to withstand the dead load of the mixers as well as torsional loads from shaft rotation and motor start-up. It also withstands axial loadings from the shaft resulting from the mixer’s impeller action.
5. Chemical feed system
Four manually actuated valves allow one pH adjustment skid (Photo 4) to serve both wastewater storage tanks, and wastewater from the tanks is recirculated by a pH recirculation pump. A programmable logic controller (PLC)-based control system automatically paces injection of acid (H2SO4) or caustic (NaOH) as necessary into the recirculation line to bring the pH into the range determined by the operator to facilitate coagulation using the selected chemical (Photo 5).
Once the pH stabilizes in the correct range, transfer pumps feed the conditioned wastewater to a coagulant and flocculant (polymer) injection system consisting of an injection point and length of serpentine piping to allow for sufficient contact between the coagulant, polymer, and suspended solids. Wastewater then enters the DAF unit where the fine bubbles described previously float the coagulated solids to the surface of the tank for removal by a mechanical skimmer. Skimmed solids drop into a sump at one end of the DAF (Photo 6). From there, they are pumped into storage vessels via an air-operated diaphragm pump. Typical solids concentrations from this process are 2 percent to 3 percent total solids by weight. The design included reuse of 400 gal (1,514 L) polyethylene totes (first used to deliver raw materials for the skincare products) as temporary sludge storage. Once full, these totes are loaded onto trucks and taken to the local municipal wastewater treatment facility where the sludge is processed through the facility’s anaerobic sludge digestion system. This system has since been enhanced and is described below.
6. Solids removal in DAF
Clarified effluent from the DAF discharges by gravity to an 8 in. (20.3 cm) diameter gravity sewer that conveys effluent from the pretreatment room to the duplex wastewater pump station, where it is pumped to the municipal wastewater collection system. DAF discharge is metered using an inline magnetic flow meter that transmits a signal back to the PLC in the process control panel (PCP). The PLC provides data logging for flow (Photo 7).
Because this facility’s process wastewater consists primarily of soap residuals, foaming was anticipated to be a problem. Therefore, an anti-foam compound is injected into the wastewater collection sump upstream of the two wastewater storage tanks.
OPERATION
7. Process control panel
As noted above, the concentrations of FOG in the wastewater can vary greatly depending on the products in production when the process equipment is cleaned. As each 10,000 gal (37,854 L) storage tank is filled, it is isolated and mixed, and the pH adjusted. Once the pH of the tank volume has stabilized, the system operators perform jar testing to determine optimal coagulation and flocculation chemical feed rates. Jar testing is done with four 1,000 ml beakers, and samples are drawn from a big sampling tub behind the pH adjustment skid. Wastewater samples are agitated, and different concentrations of coagulant are injected into each sample. Once coagulation is complete and a pin flock has formed, the operators note the coagulant dose rate for the sample that has generated the best pin flock. If a pin flock does not appear in any of the four samples, the tests are repeated at different dosage rates.
Once the coagulation test has been completed, the selected sample—the one with the best pin flock formation—is agitated gently, and different concentrations of flocculant are added to each beaker (Photo 8).
8. Jar testing (l to r) raw wastewater, flocculated wastewater, and final effluent
The sample with the best flocculation particles—the size of ¼ to ½ the size of a marble—is selected for final testing and verification. Using the dosage rates and visual information from the jar testing, the operators can program optimal coagulant and flocculant feed rates for the batch of wastewater to be processed. These feed rates are programmed into the PLC, and the treatment process is initiated. Wastewater is pumped from the isolated storage tank to the DAF unit through a chemical injection manifold. Coagulant and flocculant are injected as the wastewater flows past the injectors, and mixing takes place as the wastewater flows through serpentine piping suspended on the side of the DAF unit. This process is repeated for each 10,000 gal (37,854 L) batch and has proven efficient. Over several years, the system operators have noticed similarities in wastewater quality from specific product runs, giving them an edge when conducting the above jar testing procedure and allowing them to optimize dosages more quickly.
The controls for this pretreatment system incorporate the following interlocks to promote efficient operation and extend the operational life of the equipment:
- When liquid level in the storage tanks runs below 30 percent, the mixer will cut off
- When liquid level in the storage tanks runs below 4 ft (1.2 m), the transfer pump will cut off
- The anti-foam pump shuts down when the pH recirculation pump is not running
- Coagulant and polymer feed systems run when the transfer pump is running, shutting down when the transfer pump is off
- If chemical feed pumps are running, but no forward flow is recorded, all chemical feed pumps will shut down
SUBSEQUENT IMPROVEMENTS
After having operated this equipment for about one year, the manufacturer’s operations staff identified and implemented optimization strategies for the installed system to improve performance and address additional wastewater quality issues. These improvements are zinc removal, residuals management, and an anti-foam feed.
Zinc Removal
Sunscreen, one of the products produced at this facility, contains zinc, concentrations of which can reach 22 mg/L during production runs. While the municipality did not note this as a constituent of concern, the manufacturer recognized an opportunity to reduce this constituent with minor adjustments to the pretreatment system.
Zinc enters pretreatment in a stable aqueous form, making it unable to form as a solid. By elevating the pH to just above 8.0 in the wastewater storage tank, the manufacturer can form a zinc hydroxide (an insoluble precipitate) that can bind with coagulated FOG particles and be removed effectively by DAF. Operational data from the manufacturer shows that this process, which was not part of the original design intent, can achieve zinc removal efficiencies more than 90 percent. Note: Because copper forms a hydroxide ion at a similar pH, copper is also removed from the wastewater through this process, with a similar removal efficiency.
Residuals Management
9. Dewatering bag and tote configuration.
Sludge collected from the DAF was originally pumped with an air-operated diaphragm pump to a chemical tote repurposed for temporary sludge storage. The liquid sludge was then loaded onto trucks for transport to a municipal wastewater treatment facility with anaerobic digesters and fed through that facility’s solids stream process. The manufacturer found that the sludge totes occupied valuable floor space in the process room that could be otherwise utilized. In addition, it was looking for a way to reduce disposal costs, as most of the material being disposed of was water (sludge was averaging less than 2 percent solids).
The manufacturer’s operations staff researched low-cost options for sludge dewatering and found a materials management company that produced a bulk materials bag made from a filter cloth with a proprietary weave to promote solids retention while allowing water to pass through. The bag manufacturer noted that the bag would deflect when loaded, and that the solids retention efficiency might not be consistent. When loaded to the maximum weight of 2,205 lbs (1,000 kg), a 35 in. (89 cm) square bag may expand to between 40 and 42 in. (102 and 107 cm), a 20 percent expansion.
To address bag stretch and weave deflection, the manufacturer cut the top of a 400 gal (1,514 L) polyethylene raw product tote and placed a drainable bulk bag inside. The tote’s sides support the bag to minimize fabric deflection and maximize solids retention. This innovative approach has been effective. Water that passes through the filter cloth drains out through a bung at the bottom of the tote into a hose to the trench drain and back to the pretreatment system influent sump. This passive dewatering system results in a block of sludge that can be disposed of as solid waste rather than liquid waste. Because of the significant volume reduction through this process, waste disposal takes place far less frequently, reducing both transportation and disposal costs.
10. Dewatered sludge
Photo 9 shows the bag and tote configuration. Photo 10 shows the consistency of the dewatered sludge, with the sludge color varying depending on the dyes used in product manufacturing.
Anti-foam Feed
As noted previously, an anti-foam agent was injected into the process wastewater sump to be mixed with wastewater before transfer into the 10,000 gal (37,854 L) storage tanks. The purpose of the anti-foaming agent was to prevent foaming within the storage tank while the mixer was in operation. The manufacturer found that the performance of the anti-foaming agent was enhanced the longer it was in contact with the wastewater. As a result, the chemical injection point for the anti-foam agent was moved upstream in the wastewater collection system to be as close to the packaging equipment as possible. Mixing between the anti-foam agent and the wastewater occurs as the wastewater passes through the trench drain. This also addressed occasional foaming in the trench drain system.
PERFORMANCE
Immediately upon start-up, effluent water quality improved dramatically. After the typical start-up effluent quality variations stabilized, and the operators optimized the system, the pretreatment process consistently achieved FOG levels well below 100 mg/L. Table 1 summarizes data from the manufacturer collected over the past two years. Influent and effluent parameters represent an average of operational data collected during this period, and removal efficiencies were calculated using these averages.
CONCLUSION
The bar soap and liquid skincare product manufacturer reports that the DAF pretreatment system is robust, flexible, and reliable. The operators find that the system has been simple to operate, and that the time commitment for operation and maintenance has been minimal. In addition to the manufacturer exceeding performance goals for FOG removal, it consistently removed most of the dissolved zinc in their wastewater and cut the BOD load in the effluent by more than half, resulting in much lower monthly surcharges from the town. The manufacturer has also improved the residuals management system, reducing both the volume of residuals for disposal and the disposal costs. Adjustments for foam control also improved foam management in the trench drain system and the wastewater storage tanks.
In 2018, the manufacturer purchased an adjacent property with open manufacturing space, and is installing a second liquid skincare manufacturing facility and another similar pretreatment system.
ABOUT THE AUTHORS
- Michael Smith, PE, is a senior technical leader at Weston & Sampson in Waterbury, Vermont. He earned his Bachelor of Science in Environmental Engineering Technology from Norwich University, and has 35 years of engineering experience in wastewater facilities planning, industrial wastewater pretreatment, process design and control, systems evaluation, and construction. He also has a background with anaerobic digestion and bioenergy production. He has supported industrial wastewater pretreatment projects throughout New England.
- Josh Mandell is the engineering manager at Twincraft Skincare in Vermont. He earned his Master of Science in Mechanical Engineering from the University of Vermont and has been working in various industries from clean energy to manufacturing for the last 15 years. He specializes in project management, pilot plants, capital projects, and manufacturing engineering.
- Rachel Greene is a manufacturing engineer at Twincraft Skincare in Vermont. She earned her Bachelor of Science in Chemical Engineering from the University of Utah and has been working at Twincraft for four years. Her responsibilities include managing and overseeing the operation of the wastewater pretreatment system at its Essex, Vermont production facilities.