These systems consist of a cylindrical mix tank, agitator, acid metering pump, caustic metering pump, pH electrode/amplifier, and pH controller. The pH controller provides a proportional signal to pace the appropriate metering pump at a high pumping rate to a low pumping rate as the desired pH point is approached. The pH controllers should be field programmable. The pH electrode requires periodic cleaning, inspection, and calibration. As a neutral pH is approached on the titration curve, very small amounts of reagent will cause a large shift in pH. Therefore, proportional control is very advantageous. Generally, 50-93% sulfuric acid is used as the acid reagent. Hydrochloric acid (10-37%) is sometimes used although is less desirable due to fuming and building corrosion. Caustic soda (25-50%) is used as the base. Some designers have used magnesium hydroxide due to its safe, non-hazardous advantages. The tanks and pumps must be designed to prevent hardening and clogging as a disadvantage. Tank sizing will be considering in the discussion of continuous flow systems. The agitator is sized to provide 1-2 tank volume turnovers per minute. The metering pumps are sized to deliver the volume of chemistry for titration of the waste volume within the desired reaction time. The reagent chemistry should be delivered to the tank bottom in the path of the agitations to assure proper mixing. An injection valve should be used to avoid “over-shooting”. Tanks should be covered, sealed, and properly vented.

These systems consist of a cylindrical mix tank, agitator, gas feed system, pH electrode/amplifier, and pH controller. The pH controller provides a proportional signal to control a proportional gas valve as the pH point is approached. The pH controllers should be field programmable. These systems are unidirectional when using carbon dioxide gas to reduce a pH level. The CO2 gas is delivered to the bottom of the tank through a gas diffuser. The carbon dioxide gas forms carbonic acid to reduce the pH. The advantage of the CO2 gas is that it is safe and non-hazardous without the need for handling liquid sulfuric or hydrochloric acid. Many plants have existing CO2 gas lines within the plant. Otherwise, CO2 gas may be purchased in cylinders. A CO2 gas monitor is recommended for assuring a safe breathing environment. If a bi-directional system is required, magnesium hydroxide may be used to increase pH levels and is also non-hazardous. Tanks should be covered, sealed, and properly vented.

Most batch systems are sized to operate once or twice per day though may be designed to treat more frequently. The systems can be designed to operate manually or completely automated. A holding tank is recommended for collection of influent to provide isolation of the batch process. The operation is generally a chemical mix process as described above. The steps can be completely manual with a pH monitor or completely automated with a PLC, level control, and actuated drain valve. Most systems are semi-automatic with operator initiation and process completion check, prior to opening a discharge valve. The opportunity for a compliance check of a batch process is the advantage of the batch design. A continuous feed system has the potential to continue to discharge wastewater if out of specification if a diversion holding tank is not incorporated. This will be discussed in the continuous system section. The disadvantage of a batch treatment system is that for high flows, storage tanks can be prohibitive. Additionally, batch processes can require more labor.

A dual stage batch system can be designed for reliability and redundancy. The systems are duplicated and connected in parallel. Level controls and actuated influent valves can provide automatic start of the batch process. Automatic discharge and tank switching can be accomplished. Tank sizing depends upon the desired frequency of batch cycles and influent volume to be processed. Typically, a pH transmitter and recorder are required to be installed on the effluent drain line. Depending upon the local regulations, a written log of flow volumes may be required. A flow transmitter/totalizer and recorder may be required in lieu of a written log.

Most pH neutralization systems are designed for continuous flow since they operate 24 hours per day with minimal operator attention. These systems are fully automatic and require only periodic maintenance to replenish reagents and calibrate pH electrodes. The systems can be single stage or dual stage with two redundant systems, plumbed in series. These systems are designed as described in the Liquid Chemical Mix System section. Both systems can be enhanced with an equalization tank or pretreatment tank, upstream of the neutralization reaction tank. Downstream, a diversion tank can be provided to hold waste solution that is out-of-spec to the pumped and returned to the neutralization system. In this case, a pH monitor will signal an actuated valve to divert the waste to a holding tank and return pump until the compliance problem is corrected.

Single stage neutralization systems are typically used for waste streams of 50 gpm or less than consist of streams with a pH deviation of 3 pH units or less from the effluent range. The tank volume is typically sized for a retention time of ten to twenty minutes. For example, a 50 gpm flow with a fifteen minute retention time would have a working volume of 750 gallons. If the mixer were sized for two turnovers per minute, the pumping rate would be 1500 gpm. The influent should be directed close to the bottom by use of a down pipe or baffle to prevent short-circuiting. The effluent baffle should extend approximately one-third down from the top. Proportional pH control will pace the metering pumps to maintain the solution pH to within the proper range. Systems should be designed to handle peak flow or pH deviation conditions. Dual stage, equalization, or pretreatment system design can accommodate these conditions.

Dual stage neutralization systems are typically used for flow rates over 50 gpm. The first stage serves as the primary mix tank while the second is a trim tank. These systems provide complete system redundancy in the case of a component failure in one of the stages. The systems are generally the same size and incorporate identical design. In any case, large dumps of very highly acidic (pH 1) or highly caustic (pH 12) should not be made directly to the system. A holding tank with bleed pump could be employed if the plumbing layout will permit. Another design option would be to incorporate an equalization tank or pretreatment tank upstream.

Equalization systems are used when the pH and/or flow have peak conditions that could overload a conventional neutralization system. The equalization system includes a holding tank, mixer, level control, and transfer pump(s). The holding tank is sized to accommodate the largest dump or peak flow condition with adequate time to transfer the waste at a lower flow rate. Mixing with more dilute streams will attenuate the concentration of the wastewater. Pretreatment systems are an enhancement of the equalization concept.

Pretreatment systems consist of a holding tank, recirculation/transfer pump, level control, actuated valve, and pH control. Similar to equalization, a pretreatment system manages the influent flow based on its pH condition and volume. The recirculation pump keeps the influent solution mixed while monitoring pH. If the pH is within acceptable range, an actuated valve will transfer a portion of the flow to the neutralization reaction tank. If the pH is not within range, it will recirculate until blended and diluted by additional flow. This method allows for maintaining a larger reserve of freeboard in the pretreatment tank. Pretreatment systems are employed where concentrated dumps are possible. An overflow fitting is provided to allow gravity flow into the next stage at high level. This design concept may also be applied to the pH neutralization stages in transferring wastewater when an acceptable pH range is achieved at lower level before gravity flowing into the next stage. This design is a hybrid-automatic batch system with continuous flow. These systems are applied to conditions where wide pH swings are expected to occur.