Fail-safe neutralization of wastewater effluents

An overview of equipment, systems, and materials

There was a time when industrial facilities paid little or no attention to the waste streams from their plumbing systems. Whatever went down the sinks, toilets, or laboratory and plant system drains went into the sewers or open ponds on their property. Sometimes these wastes were piped into running streams that ran through or bordered on plant property. Out of sight, out of mind was the order of the day.

But that's all behind us now. In fact, the beginning of the end began in 1970 when the Environmental Protection Agency was established. This was soon followed with a national permit program requiring specific deadlines for the control of industrial plant discharges into the atmosphere as well as into the nation's water system.

As is the case with most political activity, regardless of intent, there followed a series of laws, regulations, and interpretations. These new constraints led to the creation and proliferation of legal specialists and compliance consultants to help plant managers understand the laws and avoid financial and corporate image penalties. This article deals with only one segment of this responsibility--the treatment of industrial wastewater to satisfy the neutralization requirements of the National Pollutant Discharge Elimination System (NPDES) and other regulations that require that fluid effluents contaminated with acids or alkalis be neutralized before being discharged into the public sewer system.

The argument that the acidic or alkaline contribution to multi-billion gallon municipal water systems by an individual industrial firm would be minuscule lost its validity when these small additions were considered collectively. Facts and figures proved rather conclusively that the combined contamination complicated controls and processing operations of the municipality, making it almost impossible for it to meet the mandated federal neutralization requirements.

Since penalties for failure are substantial, from $5,000 to $50,000 and more per day, the onus is on our backs. It is extremely important for plant
management to understand what it takes to achieve acceptable pH neutralization and the most cost-effective means for assuring fail-safe operation of the neutralization system.

Liquid waste management problems are as diverse as the industries that produce them. Whatever your plant, chances are you are vulnerable. As long as your company uses acids or caustics, the law requires you to neutralize the effluent so it conforms to local as well as federal pH requirements.

The ultimate aim of environmental regulations is to achieve the impossible: zero discharge of industrial pollutants. A more realistic approach, however, is one that requires you to meet acceptable waste standards established by the local communities. This implies that local plant managers can play an important role in determining what these standards should be in light of their plant's ability to conform, in negotiating variations from the requirements where appropriate, or in securing meaningful time-to-conform periods.

Understanding the pH requirement
Just what does it mean when the regulation requires liquid effluents to be neutral. The pH is related to the hydrogen-ion concentration in a liquid. For a solution to be truly neutral, it must be neither acidic nor alkaline. This is determined by a pH with a numerical value of 7, the value given to uncontaminated water. The neutralization process is the chemical reaction of an acid with a caustic substance that results in the formation of a salt or water. In an aqueous solution, the acid or caustic molecules disassociate and form ions. For example, with sulfuric acid (H₂SO₄) and caustic soda (NaOH), sulfuric acid is present as H⁺ and SO₄^-ions and caustic soda as Na⁺ OH^- ions. The positive H⁺ ions of the acid and the negative OH^- ions of the caustic have a strong attraction for each other and combine to form HOH = H₂O = water. This chemical process, the combination of hydrogen (H) and hydroxide (OH) to form H₂O, is referred to as neutralization. For complete neutralization, there must be a hydrogen ion for each hydroxide ion. If there is surplus of either, the solution is acid or alkaline, with a pH below 7 (acid) or above 7 (alkaline).

The significance of one number up or down is seen in the fact that the pH scale is logarithmic. Each number expresses a 10-times higher or lower concentration of the acid or alkali. The wider the acceptable range, the less costly is the system you need to install and the instrumentation it requires. When designing or installing a neutralizing system, it pays dividends not only to know the
existing regulations but the trend and the potential for change in your own
municipal district.

Neutralization systems can be either batch or continuous. For the great majority of industrial plants batch neutralization systems are the most cost-effective. Since these tend to be cyclical in nature, with volume and characteristics uniform, instrumentation is not as complex as it would be for a continuous system.

Fail-safe systems generally involve a predetermined tank size for the
gathering of the waste fluids, dosing or metering the required acid or caustic as determined by pH sensors, mixing of the neutralizing chemical with the waste fluid, controlling the fluid level in the tank, and discharging the neutralized fluid into the sewer system.

Continuous treatment systems, on the other hand, require more sophisticated pH sensing, dosing, mixing, and discharging arrangements, including downstream pH sensing systems and additional equipment and controls for recirculation and reneutralization if the discharge is not within prescribed limits. Both systems, batch or continuous, should be equipped with appropriate audible and visual alarms as well as permanent data-recording instrumentation. These are needed to ensure the discharge is in compliance and to prove that compliance to visiting government inspection teams.

Management Involvement is critical in three areas. Separate fact from fiction. Only you can get the facts on the chemicals you are handling and the waste fluids your plant is generating.

Explore the possibility of a process or chemical concentration change that might affect neutralization. Review the potentials for recycling costly chemicals.

Although this approach is not directly related to fail-safe considerations, it's a good starting point to reducing your waste neutralization problems and it helps to understand the margin for error you can tolerate. It directly affects the level of instrumentation and record-keeping you need to incorporate in your system. It also helps to pinpoint your potential liability in case of failure.

Familiarize yourself with the federal, state, and municipal regulations that affect liquid discharge into the municipal system. Take the time to meet with your state and municipal officials and don't ignore the town engineer. The more you know about local capabilities for handling liquid plant wastes, the better.

Assess the significance of the various potentials for system failure with respect to the equipment and controls you have or might have to install. These are basic to the critical management decisions that impact on fail-safe operations.

System design factors
Equipment selection is dictated by volume requirements, plant space availability and the nature of the effluents. Consideration must be given to deciding whether to locate the neutralization facility within the plant walls or to gather the waste fluids outside and conduct the processing in an outbuilding or in the open. Of equal significance are the decisions relating to the choice of construction materials. Should the system be stainless steel, glass-reinforced thermosets, or thermoplastics? Should material selection be determined by the specific fluids now being discharged or with a review of future needs or the potential for unknown or changing characteristics of the wastes?

We address these concerns by presenting a flow diagram of a typical neutralization system followed by a discussion of the critical factors related to equipment and material selection.

Self-contained neutralization systems
Figure 1 illustrates a typical neutralization system having a single pump and relatively simple controls. It's a batch type system in which the waste from process operations has been collected prior to discharging into the municipal sewer line. A non-rising rod liquid level control activates a valve in the feed line closing off or diverting the incoming waste stream when the liquid volume reaches the prescribed setting. A submersed pH sensor within the tank relays the vital hydrogen-ion concentration information to a pH analyzer equipped with two 4 to 20 milliamp output signals and a set of high and low contacts that activates the appropriate metering or dosing pump, should the pH be outside the allowable limit.

Many municipalities accept streams that are slightly alkaline, with a pH value between 7 and 9. If the liquid in the tank falls within this range, a valve in the discharge line is opened and the sump pump is energized. Should the pH be below 7--acidic--the metering or dosing pump automatically supplies a flow of caustic to neutralize the batch. As the pH rises above 7 or above the preset level, the metering or dosing pump automatically stops, and the sump pump activates. The action is identical for a pH above the preset upper alkaline level. In this case, the acid pump automatically provides the neutralizing chemical and the sump pump will be activated when the waste fluid is in the pH range acceptable for discharge. The system shown indicates a downstream pH sensor to identify and cut off the treated waste stream if it fails to fall within the acceptable range.

The equipment
Storage tanks. Mechanical engineers can readily recommend the size and structural support you need to contain the anticipated volume, the special protection against external pressure needed for in-ground tanks, and similar factors. Management has to ensure that the materials of construction are compatible with the waste fluids, that every precaution is taken to ensure leak-tightness, that double wall construction is used where hazardous materials are involved, that adequate spill containment area and necessary equipment are provided for, and that spill-handling procedures are delineated clearly.

High and low level controls. The primary function of these controls is to start and stop pumping operations automatically. Cost is related to sophistication and the need for notifying personnel of impending danger. In a single or multi-pump system, liquid level controls should activate audible alarms when fluid levels exceed the safety margin.To avoid the high cost of pump motor burnout from run dry operation, low level controls should also be set for automatic pump shut off at a designated fluid height.

The positioning of the switch housings for these controls should be well above the corrosive or hazardous fluid and sealed in watertight, and where necessary, fume-tight enclosures. In a duplex pumping system, the controls can be set to operate the pumps on alternate cycles or to operate both pumps automatically and simultaneously when incoming flow rises above the capacity of one pump to reduce the fluid height to safe operating levels.

Sensing and controls. Since pH adjustment is the most critical operation in the neutralization process, fail-safe operation is dependent on the pH sensor and the controller that activates the chemical feed pumps. The sensors selected be of highest quality and completely inert to the waste fluids. The controller must be able to initiate pump action and prevent overfeeding of either the acid or caustic pump.

Overfeeding of either chemical results in wide pH swings, unnecessary and repetitive chemical additions, and inaccurate control. This leads to discharges with unacceptable pH readings and excessive recirculation if this is picked up by a downstream pH sensor.

Pumps. Vertical centrifugal sump pumps are the type commonly recommended for use within the tanks of standard or customized neutralization pump and tank systems. The responsibility for selecting the specific design or supplier belongs with your engineering department or consultant, but there are a few questions that management should be asking.

One pertains to assurances that if plastic pumps are specified, there should be no potential for metal-to-fluid contact. Ensure that the metal shaft--usually a stainless steel or a special alloy--is completely sheathed with a thick sectioned plastic sleeve of a material compatible with the chemical waste being pumped.

Another consideration is the material from which the impeller is molded. If the fluid contains abrasive material, even though the tank is specified in polypropylene or PVC, you may want to insist that the impeller be provided in PVDF which has superior abrasion resistance. The higher cost of the
fluoropolymer will be repaid many times over by lower maintenance and less downtime.

Another significant pump design related to the handling of fluids that contain solids or stringy materials is the positioning of the impeller. Insist on a vortex, recessed impeller pump head to avoid clogging or binding. Still another costly application problem worth your concern is the potential for anticipated or unexpected dry run conditions. If this frequent cause for pump failure is present, consider the use of full cantilever bearingless designs.

The addition of caustic or acid on demand is usually handled by nonmetallic precision metering or dosing pumps. Or if accuracies of 5 percent or greater are permissible, buy less expensive rotary sealless peristaltic types that have heavy duty elastomeric flexible liners. Selection should be determined by the degree of accuracy required and the availability of the most suitable pump construction material for the chemical being metered.

Materials. Corrosion is the greatest enemy of fail-safe equipment handling process chemicals and those used for cleaning, pickling, metal finishing, surface treatment or laboratory operations. In addition, variable and mixed waste streams present a wide range of material selection problems.

This much we know. All metals corrode, some more rapidly than others. For this reason, tanks, pumps, fittings, valves, and piping for neutralizing systems are recommended in nonmetallic materials, as long as temperatures of the fluids being handled do not exceed 300 degrees F (see Figure 2).

Take the time to meet with your state and municipal elected officials and don't ignore the town engineer. The more you know about local capabilities for handling liquid plant wastes, the better.

Abrasion resistance is also critical, particularly when handling waste streams with solid particles and debris. Here, too, the nonmetallics are superior to metals. Note the wide difference in weight loss by abrasion of the different materials shown in
Figure 2. Abrasion resistance directly impacts on pump performance and dependability.

The single most common nonmetallic material recommended for pumps handling waste streams is polypropylene. This thermoplastic is chemically inert to most acids, alkalis, and solvents. It has become a standard for waste treatment and neutralizing because of its broad compatibility with diverse dilute chemicals.

Polyvinylchloride is also in wide use for handling acids and alkalis, but not for solvents. Because of its good physical properties, it has become an industry standard pipe. For the handling of strong oxidizing acids, chlorinated hydrocarbons and extremely corrosive abrasive materials, the fluoropolymers, particularly polyvinylidene fluoride, are recommended.

Fiberglass reinforced plastics are also used where the corrosive nature of the fluid permits. Because of their composite structure that uses glass fiber strands in a resin matrix, fiberglass reinforced plastic is a poor pump choice for handling abrasive liquids or slurries.

When in doubt, the safe move is to err on the side of caution. Let experience guide you as well as the corrosion tables.

The law
Regulations are written by lawyers and often unintelligible to mortals. What follows is a summary of the pertinent regulations relative to the discharge of liquid wastes. It has been prepared from data received from Jeffrey Kozel, a chemical/project engineer with Dynamac Corporation, a major environmental consulting firm whose origin coincides with the creation of the EPA in 1970. The old adage, "you can catch more flies with honey than with vinegar" is a good one to remember.

Confrontation--whether you win, lose, or draw--is seldom profitable to anyone but the lawyers. On the good news side is the fact that the government is working hard on improving its image. Discussions we have had recently with OSHA and EPA officials show a strong trend toward having their operations prove user-friendly.

Clean Water Act. Its overall purpose is to assure that the nation's waters are safe to the public and support fish and other stream life. Over the years it has been clarified and amended with regulatory and enforcement rulings calling for compliance and requiring specific discharge permits. Funds were made available by the federal government to assist municipalities in the construction of sewage treatment plants to help them meet mandated requirements.

These publicly owned treatment works were required to be in compliance within preset time periods. Since the federal monies were given to the states to enhance their role in the management and control of construction, and since the states were authorized to establish water quality standards with respect to effluent limits and compliance schedules, it is obvious that there is room for negotiation.

National Pollutant Discharge Elimination System. This program, established under Clean Water Act, refers to the discharge permits issued by EPA and the approved states. These permits are required by any source directly discharging pollutants into navigable waters. More than 60,000 permits have been issued. Indirect discharges through publicly owned treatment works are regulated under a separate program. These permits are specific with respect to average monthly and maximum daily levels, concentration and compliance schedules. Specific monitoring, testing, and reporting requirements are included. The statute was subsequently modified to include the introduction of toxic and hazardous substances into surface waters. The values were to be set in accordance with best management practices established and enforced on a case-by-case basis. The intent is to reduce secondary pollution such as those caused by raw material storage piles which require coverage against rain, and protection against run off.

Effluent guidelines. EPA was authorized to set restrictions on pollutants discharged at industrial plant outfalls. These were usually set by weight per fluid volume or by the chemical aggressiveness of the fluid expressed by its pH value.

There are three levels of technology affecting existing industrial sources.

• Best Practical Control

• Best Conventional Technology (BCT), and
• Best Available Technology Economically Achievable (BAT)

Confrontation--whether you win, lose, or draw--is seldom profitable to anyone but the lawyers.

Reporting requirements. The regulations require that discharge monitoring reports be submitted on a regular schedule. Noncompliance reports must be submitted describing the reason for the noncompliance, the expected date for return to compliance, and plans to minimize or eliminate the recurrence. If the discharge involves toxic pollutants, threat to drinking water or injury to human health, EPA must be notified within 24 hours of the event.

Pretreatment standards for
indirect discharges to publicly owned treatment works. The setting of these pretreatment standards is related to the capability of the publicly owned treatment works to accept and treat the effluent from individual industrial sources. They have full authority to regulate and control any plant effluent that might adversely affect their sewage treatment and their own compliance.

Corrosivity of the effluent is specifically listed because of its impact on piping or control instrumentation, as well as on the deleterious effect of corrosive chemicals on biological activity and human health.

Municipal and industrial stormwater permits. EPA established a stormwater permit program. The permits issued require municipalities to reduce pollutants to the maximum extent practicable for municipalities or to technology-based requirements for industry. Immediate corrective action must be taken when a discharge contributes to a violation of a water quality standard or is a significant contributor of pollutants to the nation's waters.