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Waste Neutralization34a_m11_r0

WASTE NEUTRALIZATION

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Contents

11.1 Fundamentals ......................................................................................................... 2

11.2 Process and Operation Overview ....................................................................... 3 11.2.1 Wastewater Collection ................................................................................ 3 11.2.2 Wastewater Mixing ...................................................................................... 3 11.2.3 pH Measurement ......................................................................................... 5 11.2.4 Wastewater pH Adjustment ....................................................................... 6

11.3 Equipment Design and Options ......................................................................... 7 11.3.1 Batch Tank ..................................................................................................... 7 11.3.2 Tank Mixing Eductor ................................................................................... 7 11.3.3 Centrifugal Pumps ...................................................................................... 8 11.3.4 pH Sensor ...................................................................................................... 9 11.3.5 Valves and Piping ........................................................................................ 9

11.4 Application and Design ..................................................................................... 10 11.4.1 Application.................................................................................................. 10 11.4.2 Design Calculations .................................................................................. 10 11.4.3 Expected Results ......................................................................................... 13

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34a_m11_r0 Waste Neutralizationtable of contents

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Neutralization

AcceptablepH forDischargeto Drain

Figure 11.1-1: Acidic and Alkaline Wastewaters Neutralize Each Other

AlkalineWastewater

AcidicWastewater

34a_m11_r0table of contents

Waste Neutralization11-2

Module 11WASTE NEUTRALIZATION

11.1 Fundamentals

Neutralization involves adjusting the pH of a liquid to approach the “neutral” pH of7.0 (neither acid nor base). The concept of pH is discussed in detail in Module 2,Section 7. Generally, neutralization involves the use of an acid (pH less than 7) tolower the pH of a tank of basic (or alkaline) liquid (pH greater than 7), or the use of abase (or alkali) to raise the pH of a tank of acidic liquid.

In the water treatment industry, wastewater is generated from the regeneration ofthe resins used in cation, anion, and mixed bed ion exchange systems. The chemicalsused to regenerate the resins have extreme pH levels. For example, sulfuric acid,with a pH of 1-2, is used for regeneration of cation resin and sodium hydroxide, witha pH of 13-14, is used for regeneration of the anion resin. After the regenerationprocess, the water containing these chemicals must be disposed of. Due to its ex-treme pH, the wastewater cannot be sent directly to drain, as this will typically vio-late local regulations regarding the pH of wastewater discharge. For example, theNPDES (National Pollutant Discharge Elimination System) standard for streamdischarge pH is 6-9. The acceptable pH of the discharge varies by location and thedestination of the discharge (such as a stream, a well or a sewer leading to amunicipal waste treatment plant).

To neutralize the highly acidic wastewater from a cation resin regeneration, a supplyof alkaline liquid is needed. An anion resin regeneration is usually performed at thesame time, and its wastewater is alkaline. When these two wastewater streams arecombined, the pH is “neutralized” and approaches 7. If the resulting pH of thecombined liquids is not within the acceptable range for discharge, an additionalamount of either acid or base must be added to shift the pH into the acceptablerange. Figure 11.1-1 on the opposite page shows this concept. Generally, thechemicals used to adjust the wastewater are the same chemicals used to regeneratethe resins in the ion exchange systems.

The regeneration of resin from a mixed bed ion exchange system generates bothhighly acidic wastewater and highly basic wastewater. The total amount ofwastewater generated is generally a lower volume than the combined wastewaterstreams from a pair of cation and anion ion exchange systems. Regardless of thevolume, the two wastewater streams resulting from the regeneration of a mixed bedsystem can be combined to nearly neutralize each other.


Figure 11.2-1: Batch Neutralization Tank

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BatchNeutralizationTank



11.2 Process and Operation Overview

The process of neutralization involves the following:

• Collection of wastewater resulting from the regeneration of various ionexchange systems

• Combining and mixing wastewater streams

• Measuring the pH of the combined wastewater streams

• Adjusting the pH of the wastewater so it is within acceptable limits fordischarge to drain

11.2.1 Wastewater Collection

The wastewater remaining at the conclusion of an ion exchange resin regenerationcycle generally has an extreme pH and cannot simply be sent to drain. Instead thewastewater is directed to a “batch neutralization” tank. Figure 11.2-1 on theopposite page shows a typical batch neutralization tank. The purpose of the tank isto hold the wastewater streams in one location. Once collected, the goal is to use thewide variations in pH of the streams and cause them to neutralize each other.

11.2.2 Wastewater Mixing

For complete neutralization to occur in a reasonable amount of time, the acidic andalkaline waste volumes in the batch tank must be thoroughly mixed. There areseveral mixing approaches that can be considered.

A motor-driven mixer on a shaft can be used to mix the contents of the tank. Thetall tanks often used in this application necessitate the use of long shafts that requirecareful balancing with submerged bearings to keep the shafts in place. Mountingthe mixer assembly on the side of the tank removes these requirements but adds anunderwater seal. The maintenance for either of these approaches can be difficult.

Mixing can also be achieved by blowing air into the bottom of the neutralizationtank. This method avoids the use of moving parts and the associated maintenanceconcerns, but requires a suitable supply of air. This procedure usually requires alarge set of blowers to produce the volume and pressure of air required to mix a fulltank of water.

Waste Neutralization34a_m11_r0table of contents

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Another mixing method lets centrifugal pumps recycle the wastewater in the tank.This approach provides a fast and efficient mix with a relatively low-maintenancepiece of equipment. Only one pump operates, and any additional pumps are forredundancy. Figure 11.2-2 below shows a pair of recycle pumps.

Figure 11.2-2: Recycle / Discharge Pumps

In a recycle mixing design, water flows from the bottom of the neutralization tankand proceeds to the suction of the operating recycle pump. After being dischargedfrom the pump, the water is directed back into the batch tank. The time required topump the entire contents of the tank one time is the “turnover rate.” This can bemeasured in turns per hour, with one turn being equal to the volume of the batchtank flowing through the pump one time.

Recycle/Discharge Pumps


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Figure 11.2-4: Principle of Tank Mixing Eductor

One gallonpumped

..Five gallonscirculated

Suction

SuctionNozzle

ParallelSection



As it enters the tank, the water passes through an “eductor” that mixes water fromthe pump (the motive fluid) and water from another part of the tank (the entrainedfluid). The batch tank contains a series of eductors, equally spaced on laterals, tothoroughly mix the tank’s contents. Figure 11.2-3 below shows the arrangement ofeductors inside the batch tank.

Figure 11.2-3: Tank Mixing Eductors

The flow of water from the recycle pump through an eductor creates suction thatpulls in approximately four times the amount of water being pumped. For everygallon pumped into a single tank mixing eductor, five gallons are discharged, whichsignificantly reduces the turnover rate. Figure 11.2-4 on the opposite page shows aschematic of how an individual eductor functions. The agitation caused by thedischarge stream also encourages mixing of the tank contents, especially when usinga series of eductors arrayed through the batch tank.

11.2.3 pH Measurement

While the wastewater is being recycled, an inline sensor continuously monitors itspH. As the wastewater in the tank is being mixed, the pH is checked to verify that itis within acceptable limits for discharge. If the pH of the batch is acceptable, thewater is sent to drain. If the pH of the batch is outside the acceptable range, addi-tional chemicals are added to bring the pH to a desirable level before it is discharged.

Tank Mixing Eductors


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11.2.4 Wastewater pH Adjustment

If the batch of mixed wastewater from a cation resin regeneration and an anion resinregeneration does not yield an acceptable pH for discharge, then the batch must beadjusted. Based on the pH measurement, an algorithm in the control systemdetermines the amount of acid or caustic that must be added to the batch tank.Figure 11.2-5 below shows where the chemicals are added. If the pH of the batch istoo high, acid is added. If the pH of the batch is too low, caustic is added. Theamount added should be sufficient to bring the pH of the batch tank contents intothe acceptable range. The chemicals used are commonly the same chemicals usedfor cation or anion resin regeneration.

Figure 11.2-5: Location of Chemical Additions

After the adjustment chemical is added and the contents of the tank are thoroughlymixed, the pH of the wastewater batch is measured again to confirm that it is withinthe acceptable range. If the pH is acceptable, the contents of the batch neutralizationtank are sent to drain. If the pH is not acceptable, another adjustment cycle isexecuted until the pH of the batch is acceptable.

CausticAcid


Figure 11.3-1: Batch Neutralization Tank

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BatchNeutralizationTank


11.3 Equipment Design and Options

The equipment for a waste neutralization system is relatively straightforward. Dueto the extreme pH of the wastewater and the adjustment chemicals, the choice ofconstruction materials is critical.

11.3.1 Batch Tank

The batch tank is an atmospheric tank designed to withstand the extreme pH of thecation and anion resin wastewater. Figure 11.3-1 on the opposite page shows thebatch tank. The tank itself is made of either fiberglass (FRP) or carbon steel. Due tothe extreme pH of the wastewater streams, a carbon steel tank is lined with an epoxypolyamide sprayed lining or natural rubber sheet lining. For more information onatmospheric tanks, see Section 9.1.

11.3.2 Tank Mixing Eductor

A series of eductors are used to mix the contents of the batch tank in a shorteramount of time than required by a traditional agitator. By using the flow of waterthrough the eductors to create suction, water is drawn into the eductor from otherparts of the tank and combined with the water from the recycle pump. Typically, forevery gallon of water pumped into an individual eductor, four additional gallons aredrawn in to be mixed. This creates a circulation ratio of 5:1; five gallons leave theeductor for every one that is pumped in. Figure 11.3-2 below shows a single tankmixing eductor.

For good mixing, the discharge plumeof the eductor should cover most ofthe height of the tank. An eductortypically provides a discharge plume(in feet) of about half the drivingpressure (in psi). Since tanks areusually taller than they are wide, andeductors are mounted at an angle, a 60psig driving pressure is typically used.

Figure 11.3-2: Tank Mixing Eductor

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11.3.3 Centrifugal Pumps

The centrifugal pumps supply the mixing energy for the system. They can also beused to send the neutralized tank contents to drain. Figure 11.3-3 below shows atypical centrifugal pump arrangement.

Figure 11.3-3: Centrifugal Pumps

The pump materials of construction must be able to withstand the pH extremes ofthe wastewater and the chemicals present. For more information on centrifugalpumps, see Section 9.2.

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MotorPump


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11.3.4 pH Sensor

The inline pH sensor monitors the pH of the wastewater. The sensor measures theelectrical potential between a reference material and the wastewater flowing past an

inline probe. The signal isconverted to electricalcurrent and transmitted tothe control system. Theresults are used todetermine whether thewastewater batch iswithin an acceptablerange for discharge. Italso monitors the batch asit is being discharged. Ifthe pH is not acceptable,the control systemdetermines what type ofadjustment chemicalshould be added to bringthe batch into theacceptable range.Figure 11.3-4 shows pHprobes and associatedelectronics.

Figure 11.3-4: Inline pH Monitor

11.3.5 Valves and Piping

Like other components of the waste neutralization system, the valves and pipingmust be able to withstand the extremes in pH of the incoming wastewater streams.Stainless steel is a typical choice as the material of construction due to its corrosionresistance. However, if hydrochloric acid (HCl) is used as an adjustment chemical,components that come in contact with the concentrated acid must be protected(due to the corrosive effects of high chloride levels). If carbon steel is used, it mustbe lined with polypropylene (PPL) or a Teflon-type material (TFE). PVC pipe is oftena good choice in sizes smaller than 8 inches. Larger size PVC fittings tend to befragile. Similarly, piping just downstream of a sulfuric acid injection point must beprotected from the heat generated by the dilution reaction. Plastic pipe is not a wisechoice in this application.

ReferenceMaterial Display

In-lineProbes


Figure 11.4-1: Waste Neutralization System Calculated Attributes

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Batch TankVolume

EductorQuantityand Size

PumpSize



11.4 Application and Design

11.4.1 Application

A waste neutralization system is generally used when wastewaters with extreme pHvalues are generated by the regeneration of resins in ion exchange systems. If thefacility or municipal waste treatment systems cannot handle the wide variations inpH along with the peak flows, a neutralization system is required.

11.4.2 Design Calculations

Figure 11.4-1 on the opposite page shows the components of a waste neutralizationsystem for which key attributes must be calculated. The following input data areassumed:

• The amount of wastewater (in resin volumes) generated by each resinregeneration step:

ANION Volumes CATION VolumesBackwash 2 Backwash 2Pre-Heat 1 - -Caustic 2 Acid 2Displace 1 Displace 1Rinse 10 Rinse 6Total volumes 16 Total volumes 11

Volumes could be lower depending on the ion exchange vessel andwhether rinse water is recycled. Actual volumes of waste produced arenormally calculated with detailed ion exchange process calculations.

• The desired turnover rate. (Assume 4 turnovers to thoroughly mix the batchand 10 minutes per turnover to quickly process the batch.)


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Batch Tank Volume

To use the wastewater from an anion resin regeneration to neutralize the wastewaterfrom a cation resin regeneration, the batch tank must be able to hold all of thewastewater from the regeneration of both systems. A safety factor of 50% is addedto this total to account for conditions (for example, regeneration cycle alarms) thatmight increase the amount of wastewater generated. The batch tank must alsoinclude extra volume for the low level cut-off alarm (5% of total), high level alarm(5% of total) and overflow alarm (5% of total).The total volume of the batch tank is calculated as follows:

VTotal = (Vcation waste + Vanion waste ) x 1.5 [safety factors] x 1.15 [total alarm factors}

Where:Vcation waste = cation resin volume x waste volume generated per cation resin volumeVanion waste = anion resin volume x waste volume generated per anion resin volume

The calculations must be done in consistent units, usually cubic feet (as resin vol-umes are mostly expressed in cubic feet) then converted to U.S. gallons using theconversion factor 7.5 U.S. gallons/cubic foot.

For a system with 170 cubic feet of cation resin and 226 cubic feet of anion resin pervessel:

Vcation waste = 170 ft3 x 11 ft3 waste per ft3 resin = 1,870 ft3 wasteVanion waste = 226 ft3 x 16 ft3 waste per ft3 resin = 3,616 ft3 waste

VTotal= (1,870 ft3 + 3,616 ft3) x 1.5 x 1.15 = 9,463.35 ft3

Converting to US gallons:

VTotal= 9,463.35 ft3 x 7.5 USgal/ft3= 70,975 US gallons (or 71,000 gallons)

The height and diameter of the tank can be adjusted to accommodate this volumeand the location of the waste neutralization system.


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Eductor Quantity and Size

Eductors are available in a range of sizes, however, larger sizes tend to be machinedand thus quite expensive. If small units are selected, the installation cost tends to behigh because of the large quantity required. Currently, a 1 ½” unit appears to be thelargest standard size available at a cost-effective price. Table 11.4-1 below providesthe operating characteristics of this unit as a function of operating pressure. Notethat, at all pressures, the circulation ratio is very close to 5 gpm of circulation per gpmof motive flow.

Table 11.4-1: Elmridge 1½ inch Eductor Capacities

The mixing flow, or total eductor capacity, is calculated by dividing the tank volumeby the desired turnover time:

Mixing flow (gpm) = VTotal (gal) / turnover time (minutes)

The mixing flow is divided by the eductor circulation ratio to determine the actualflow to the eductors, called the motive flow:

Motive flow (gpm) = mixing flow (gpm) / circulation ratio

The circulation ratio depends on the eductor selected but usually varies only slightlyacross a specific eductor product line.

The minimum number of eductors required is calculated by dividing the total motiveflow by the motive flow per eductor at the operating pressure selected, and roundingup to the next integer. Mechanical layout considerations may increase this numberslightly but do not need to be considered during process calculations with standardeductors.

Eductor Model ME4OP (1.5 inch)

Pump Pressure(psi)

10 15 20 25 30 35 40 50 60

Motive Flow(gpm)

33 40 47 52 57 62 66 74 81

Circulating Flow(gpm)

165 202 233 261 286 309 330 369 404


Continuing with the sample system and using the 10-minute turnover time initiallyselected, the values for mixing flow and motive flow can be calculated as follows:

Mixing flow = 71,000 gallons / 10 minutes = 7,100 gpmMotive flow = 7,100 gpm / 5 = 1,420 gpm

At 60 psi driving pressure, each 1 ½” eductor uses 81 gpm of water, yielding a totaleductor count of 18:

Eductor count = 1,420 / 81 = 17.53, or 18

Pump Size

The pump size is fixed by the number of eductors selected (above). The pump flowequals the number of eductors times the motive flow per eductor. The pump pres-sure is the operating pressure (normally 60 psig) selected to get a reasonable plumeheight.

For the example above:

Pump flow = 18 eductors x 81 gpm per eductor = 1,458 gpmPump head = 60 psig x 2.3 ft TDH per psig = 138 ft TDH.

11.4.3 Expected Results

After mixing and potential adjustments, the contents of the batch neutralization tankwill have a pH within local acceptable limits for discharge to the wastewater’s finaldestination (stream, well, or sewer leading to a municipal waste treatment plant).

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