P.1 

OAR Box 1158 

Prepped by RyanDugan 

Document Number: 

65) ll-A-9 

Docket Number: 

A-88-02 

Printed 4/28/2008 2:34:14 PM Sheet 65 of 349 
»EPA 

United States 

inv.ironme.ntal Protection 

Agency 

Technology Transfer 

EPA/625/5-85/016 

September 1985 

Revised 

P.2 

Environmental Pollution £T?2 

Control Alternatives 

Reducing Water Pollution 

Control Costs in the 

Electroplating IndustwHjjjjSJLJiJ 

JAN I 8 

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P.3 

Technology Transfer EPA/625/5-85/01 ( 

Environmental Pollution 

Control Alternatives 

Reducing Water Pollution 

Control Costs in the 

Electroplating Industry 

September 1985 

This report was prepared jointly by 

Industrial Technology Oivision 

Office of Water Regulations and Standards 

Office of Water 

Washington, DC 20460 

and 

Center for Environmental Research Information 

Office of Research Program Management 

Office of Research and Development 

Cincinnati, OH 45268 

Z-
P.4 

Cyanide waste 

Caustic - i 

Acid-i 

Caustic 

Chrome waste 

Acid/alkali waste 

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... V •>•_«_• £La.-

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Polymer 

t 

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•••S&iW 

f l Wastewater 

discharge 

Neutralization 

precipitation 

Rocculation 

Averaging 

Filter cake .^ ^ 

to approved 

disposal 

Sludge 

thickening 

Filtration 

Rgure 1. Conventional Wastewater Treatment System for Electroplating 

Capital Costs 

The unit processes shown in Figure 1 

are used extensively in the 

electroplating industry, and as a 

result their design has become 

somewhat standard. The ability to 

standardize has reduced the high 

cost of site preparation and 

construction through the 

development of skidmounted 

package systems, complete with all 

hardware and auxiliaries. Installation 

costs for package systems usually 

range between 10 and 30 percent of 

the purchase price ofthe equipment 

compared with installation costs of 

50 to 100 percent of purchase price 

for component systems. 

The costs presented in the following 

sections assume the purchase of all 

components of the individual 

systems. Costs could be reduced by 

using existing pumps, tanks, and 

instrumentation, but these are rarely 

available. Installation costs will 

increase as a result of site-specific 

requirements related to wastewater 

collection systems, new building 

space, structural modifications, or 

the relocation of existing equipment. 

Wastewater Collection. Wastewater 

from individual plating operations 

must be directed to the appropriate 

treatment components, which often 

entails separate wastewater 

collection systems for chromium, 

cyanide, and acid/alkali wastes. 

Typically, a collection pipe system is 

installed parallel to the plating line 

so that spent rinse waters can be 

drained to one or more treatment 

sumps. Equipment washdown and 

spilled materials are collected in a 

trench which also leads to the 

appropriate sump. The treatment 

sump is equipped with a level-
P.5 

controlled pump that delivers the 

wastewater to the treatment unit. In 

addition, some provision must be 

made for collecting the strong 

cleaning and plating chemical 

solutions which are usually bled at a 

slow, constant rate into the 

treatment system by way of an 

averaging tank to avoid upsetting the 

process. 

Wastewater collection costs are 

highly variable. A plant may have to 

provide as many as five separate 

collection/sump systems to handle 

three different kinds of wastewater 

plus spent cyanide and noncyanide 

chemical solutions. Cost 

components for wastewater 

collection are given in Table 3. 

Larger sumps are used for chromium 

and cyanide wastewater collection; 

in fact, the sump itself may be used 

as the treatment vessel (this practice 

is rare). It should be pointed out that 

custom-made concrete sumps are 

considerably more expensive to 

construct than the preformed sumps 

indicated in Table 3. 

Table 3. 

Costs for Wastewater Collection 

Collection conduit:" 

Linear runs: 4"-$1.50/ft 

6"-$2J0/ft 

4"-$200.00 

6"-$350.00 

4"-$200.00 

6"-$300.00 

Each connection: 

Each bend: 

Collection sumps:' 100 gallons ...$2,000 

300 gallons ... $2,500 

500 gallons .. . $3,000 

1,000gallons ... $3,500 

°PVC pipe. 

"Preformed PVC inground tank with sump 

pump, level control and steel grating cover. 

1984 data. 

Chromium Reduction. Chromium is 

usually present in electroplating 

wastewater as trivalent cnromium 

(Cr+3) or as hexavalent chromium 

(Cr+$). Although most heavy metals 

are precipitated readily as insoluble 

hydroxides in the neutralizer, 

hexavalent chromium must first be 

reduced to trivalent chromium. 

Reduction is usually achieved by 

reaction with gaseous sulfur dioxide 

(S02) or a bisulfite solution 

(NaHS03). Using sodium 

metabisulfite (Na2S2Os), the net 

reaction involves chromic acid and 

sulfuric acid in the formation of 

sodium sulfate, chromium sulfate, 

and water: 

3Na2S2Os + 4H2Cr04 + 3Hs>S04-» 

3Na2S04 + 2Cr2(S04)3 + 7H20 

Because the reaction proceeds 

rapidly at low pH, an acid is added to 

maintain the chromic acid 

wastewater between a pH of 2 and 3. 

To prevent the release of sulfur 

dioxide during the reaction, 

maintaining the pH near 3 is advised. 

In addition to the use of sulfur 

dioxide or bisulfites to reduce 

hexavalent chromium, there are 

three commercially available 

methods that use the reducing 

potential of iron and/or iron salts. 

Ferrous sulfate reduces hexavalent 

chromium according to the following 

equation: 

3Fe + 2 + Cr + 6 . 3Fe+3 + Cr + 3 

The use of iron in this form adds 

considerably to the volume of sludge 

produced by the treatment system. 

This method is rarely used except 

where an effluent of high chromium 

content is located close to a cheap 

and abundant source of ferrous salts, 

such as waste pickle liquor from a 

steel mill. 

Ferrous salts may also be generated 

by a patented process employing an 

electrolytic cell having iron anodes 

and inert cathodes: 

3Fe° + 6e" 3Fe + 2 

As the iron anodes are dissolved in 

the process, they must be replaced 

with new ones. While this process 

will also produce substantial 

quantities of excess sludge, it 

provides a convenient method of 

continuously treating low 

concentrations of hexavalent 

chromium, such as might be found 

in cooling tower blow-down waters 

or in rinses from chromate 

conversion coatings. With higher 

chromium concentrations it is 

difficult to economically produce the 

required ferrous ion concentrations 

on a flow-through basis. 
P.6 

so r 

20 30 40 

FLOW RATE (gal/min) 

50 60 

Legend: 

_—•» - Total installed cost 

- TM" " Hardware cost 

Notes: 

Batch units: 

Installed cost = 2 x hardware cost. 

Unit consists of two 4-hour reaction tanks 

with necessary auxiliaries. 

Continuous units: 

Installed cost = 1.25 x hardware cost. 

Unit consists of equipment shown in Figure 1. 

Rgure 2. Investment Cost for Chromium Reduction Units 

Iron, in the form of small pieces of 

scrap steel, may also be used to 

directly reduce hexavalent 

chromium. This patented process is 

controlled by adjusting the pH ofthe 

influent according to its hexavalent 

chromium content. A pH of 2.0 to 2.2 

will usually accommodate chromium 

loadings as high as 200 to 300 ppm. 

Lower pH settings would be required 

to treat higher chromium 

concentrations. The following 

equation represents the reduction 

mechanism: 

Fe° + Cr + 6 Fe+3

 + Cr + 3 

Because the iron and chromium 

react on a one-to-one basis, rather 

than a three-to-one ratio, the sludg _ 

produced by this process is 

significantly less than that generated 

in the other processes using iron. 

The low cost of scrap steel allows 

this process to compete favorably 

with the cost of sulfur dioxide 

treatment, while eliminating the 

potential of releasing hazardous gas. 

Hardware and installation costs from 

packaged continuous and batch 

chromium reduction units are shown 

in Figure 2. Costs include storage 

and feed systems for the treatment 

reagents, as well as the costs for 

hardware, piping, instrumentation, 

and utility connections. The 

continuous unit costs are based on 

using sodium metabisuifite as the 

reducing agent and a wastewater 

retention volume of 30 min. For very 

small flows, simpler and less costly 

batch systems are feasible. The 

batch system costs include two 

reaction tanks each sized to hold 4 h 

of wastewater flow and equipped 

with high level alarms, portable pH 

and oxidation reduction potential 

(ORP) meters, a portable mixer, and 

storage tanks and feed pumps to add 

sodium metabisuifite and sulfuric 

acid to the reaction tanks. 
P.7 

Cyanide Oxidation. Dilute cyanide 

rinse streams resulting from plating 

operations and cyanide dips must 

also be treated separately to oxidize 

the highly toxic cyanide, first to less 

toxic cyanate, then to harmless 

bicarbonates and nitrogen. The 

oxidizing reagent is usually chlorine 

gas (Cl2) or sodium hypochlorite 

(NaOCI). Using chlorine, the typical 

reaction in the first stage involves 

sodium cyanide (NaCN) and sodium 

hydroxide (NaOH): 

Cl2 + NaCN + 2NaOH-+ 

NaCNO + 2NaCI + H20 

and in the second stage: 

3CI2 + 2NaCNO + 5NaOH -* 

6NaCI + C02 + N2 + 

NaHC03 + 2H20 

When sodium hypochlorite is used, 

the typical reaction in the first stage 

is: 

NaOCI + NaCN -* NaCNO + NaCl 

and in the second stage, 

3NaOCI + 2NaCNO + H20-> 

3NaCI + N2 + 2NaHC03 

In most continuous systems, it is 

preferable to conduct the operations 

in two series-connected reaction 

tanks rather than in stages in one 

tank. In the first stage, the pH is 

adjusted between 9 and 11 using an 

alkali such as caustic soda or lime. 

The pH in the second reaction 

chamber is controlled to 

approximately 8.5. Sodium 

hypochlorite is added continuously 

to both stages. Demand in each 

stage is determined by measuring 

ORP. The reaction time needed is 

approximately 30 to 60 min in each 

stage. 

Figure 3 shows cost curves for 

continuous and manual batch 

cyanide oxidation units. The 

continuous unit cost is for a unit 

which uses sodium hypochlorite as 

the oxidizing agent. The cost 

includes storage and feed systems 

forthe treatment reagents. The batch 

system cost is for a system with two 

4 h reaction tanks and the auxiliaries 

required to add sodium hypochlorite 

and control pH. 

Again, for very small flows, simpler 

and less costly batch systems are 

feasible. At wastewater flow rates 

below 20 gal/min (76 l/min) batch 

units appear to be more costeffective than continuous units. 

50 

40 

o 

o 

o 

530 

tO 

<J 

rZ 

UJ 

5 

ttn 20 

UJ > 

Z 

10 

0 

TM 

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^^^ 

^ ^Xl/"  units ^ ^ 

rlTy^ *<? ^ <T ^ 

J ' ' // ** 

y > 

/^ Batch units 

f

 , ' y s 

s 

I I i I I 

10 20 30 40 50 

FLOW RATE (gal/min) 

Legend: 

1 , • Total i .'stalled cost 

= • = = Hardware cost 

Notes: 

Batch units: 

Installed cost = 2 x hardware cost. 

Unit consists of two 4-hour reaction 

tanks with necessary auxiliaries. 

Continuous units: 

Installed cost = 1.25 x hardware cost. 

Unit consists of equipment shown ih 

Figure 1. 

Figure 3. Investment Cost for Cyanide Oxidation Units 

(o 
P.8 

Neutralization/Precipitation. Waste 

streams from each of the various 

metal cleaning and plating 

operations are combined in the 

neutralizer with the effluent from the 

chromium reduction and cyanide 

oxidation units. Because the heavy 

metals are only soluble under acidic 

conditions, the pH is adjusted to a 

range of 9.0 to 9.5, at which the 

metals precipitate as hydroxides. 

Afterthe pH is adjusted, a small 

amount (1 to 5 mg/l) of a coagulating 

agent such as aluminum sulfate, 

ferrous sulfate, or calcium chloride is 

added to the wastewater to facilitate 

precipitation. 

Many types of neutralization 

systems can be designed with 

various degrees of automation and 

controls, depending on the 

magnitude and variability of the flow 

and its pH. Because a change of 1 pH 

unit represents a tenfold change in 

hydrogen ion concentration and a 

hundredfold change in solubility for 

some metals, maintaining the pH in 

the narrow range where maximum 

removal of pollutants is realized is 

critical, though difficult, especially 

when the neutralizer feed is subject 

to wide variation. 

A single-stage, continuous 

neutralizer, in which all the alkali 

such as lime or caustic soda is fed 

into a single reaction vessel, is 

suitable for most electroplating 

applications. If the wastewater is 

subject to rapid changes in flow rate 

or pH, however, a multistage 

neutralizer is required. In the 

multistage units, most of the alkali is 

added in the first vessel to increase 

the pH to 6. Final pH adjustments of 

the wastewater are made in the 

remaining reaction vessels to 

promote precipitation and to 

enhance the settling characteristics 

ofthe metal hydroxides. To maintain 

adequate pH control, the retention 

time for typical neutralization is 15 to 

30 min. The efficient use of lime 

requires a minimum of 30 min 

because lime reacts more slowly 

than sodium hydroxide. 

Figure 4 shows hardware and total 

installed costs for a continuous 

neutralization unit typically used in 

the electroplating industry. This unit 

is single stage with pH-controlled 

addition of caustic soda and sulfuric 

acid. 

40 

__. 30 

o 

tn 

O 

u 20 

> 

z 

10 

Total installed cost 

I 

I 20 40 60 

FLOW RATE (gal/min) 

Notes: 

Installed cost = 1.25 x hardware cost. 

Cost does not include flocculation system. 

80 100 

Figure 4. Investment Cost for Continuous Single-Stage Neutralization/ 

Precipitation Unit 

1 
Clarification. Metal hydroxides and 

other insoluble pollutants are 

removed from the wastewater by 

gravity settling and/or filtration. The 

removal efficiency depends on the 

settling rate of the suspended solids 

in the wastewater feed. Typically, 

some of these solids settle very 

slowly because of their small size 

and their slight density difference 

compared with the water. Because 

an economical design ofthe clarifier 

limits the retention time in the 

settling chamber, some level of 

suspended solids will appear in the 

overflow. 

60 

50 

S 4 0 

o 

o 

tV) 

O INVESTMENT C 

Ti ro to O o o 

0 

-

i i 

20 40 

To enhance the settling 

characteristics of the suspended 

solids, a flocculating agent, usually 

an organic polymer, is added in a 

fiocculator where the wastewater is 

agitated gently to allow the solids to 

agglomerate. The wastewater then 

enters the clarifier where the solids 

settle out. The solids in the 

underflow can be discharged to a 

holding tank for subsequent 

dewatering. 

Total installed cost 

^^r ^ Hardware cost 

P.9 

1 

The optimal doses of flocculating 

agents, hardware specifications, and 

solids removal efficiencies are 

usually estimated based on 

laboratory tests conducted by the 

equipment vendor. Figure 5 shows 

hardware and total installed costs for 

flocculation and clarification units 

typical of those used in the industry. 

These costs are presented as a 

function of volumetric flow rate; they 

also reflect the effect on unit costs of 

solids-settling rates and the level of 

solids allowed in the effluent. The 

units are assumed to have a separate 

flocculation tank, a polymer feed 

system, a "lamella" (or slant-tube 

separator), and a zone in which 

sludge collects before being 

discharged. These units are more 

widely used than simpler rectangular 

settling chambers. 

Notes: 

Installed cost = 1.25 x hardware cost. 

Cost includes plate-type clarifier with flocculating 

l 1 1 l ch 

60 80 100 120 

FLOW RATE (gal/min) 

:

igure 5. Investment Cost for Flocculation/Clarification Units 

10 

amber and polymer feed system. 

: 

8 

.-J 
P.10 

Manually shifted low-pressure recessed-plate filter press with capabilities for filling disposal drums directly. 

Sludge Handling. The solids from 

clarifiers are typically discharged to 

sludge holding tanks at solids 

concentrations of 0.5 to 3%; 

overflow from the tank is recycled to 

the clarifier or to the flocculation 

tank. Usually, metal hydroxide solids 

will concentrate to approximately 3 

to 5% solids in the sludge holding 

tanks if sufficient retention time is 

allowed. The tanks must also provide 

adequate volume to store the sludge 

before it is shipped to a disposal site 

or transferred to another dewatering 

stage. Figure 6 shows the investment 

for sludge holding tanks as a 

function of tank volume. 

Further concentration of the 

thickened sludge requires the use of 

mechanical dewatering equipment. 

Centrifuges, rotary vacuum filters, 

belt filters, and filter presses have 

been used to dewater metal 

hydroxide sludge. The applicability 

of a particular dewatering device for 

a specific sludge, and the degree of 

cake dryness the device will achieve, 

can be determined by bench-scale 

tests conducted by the vendor using 

the intended feed material. 

Figure 7 presents the unit costs for 

recessed plate filter presses as a 

function of the filter-cake volume of 

the unit. The feed volume capacity of 

the unit is also given, based on a 

feed solids concentration of 2%, a 

cake solids concentration of 20%, 

and a press cycle of 8 h. The costs 

shown in Figure 7 do not include 

installation or auxiliary equipment 

associated with the press, because 

these costs are highly variable and 

site-specific. Items that will 

contribute to the cost of installation 

include: 

• High pressure feed pump(s) (often 

quoted and supplied with the 

filter) 

• Sludge feed storage tanks 

• Filtrate return lines (to clarifier) 

• Cake solids handling equipment. 

11 

q 
P.11 

30 

o o 

5

2

° 

OT 

. O INVESTMENT 

o 

0 

t 

1 1 1 

1000 2000 3000 

VOLUME (gal) 

Total investment cost 

Hardware cost 

I I 

4000 5000 

Notes: 

Based on carbon steel construction. 

Costs include fiber-reinforced polyester 

tank and diaphragm pump. 

Rgure 6. Investment Cost for Sludge Storage/Thickening Units 

400 

=5 300 ME(g 

D 

O 

> 200 

UJ 

_«_ 

< 

cc 

H 

E 100 

0 

y 

/ ' 

/ 

/ 

f Manual 

" / 

lll l 

10 20 30 40 

EQUIPMENT COST ($1,000) 

/ 

/ 

/ 

" Automatic 

1 

50 6 

500 

375 

gal/h) 

2 

250- Z. 

—i 

O 

> 

U l 

LL. 

125 

D 

Notes: 

Cost includes carbon steel frame, polypropylene plates and filter cloths. 

Installation costs not included. 

Cake volume based on 1 VV thick sludge 

cakes. 

Feed volume capacity based on: feed 

solids = 2%; cake solids = 20%; cycle 

time = 8 hours. 

Figure 7. Hardware Cost for Recessed Plate Filter Presses 

12 \ 0 
P.12 

Operating Costs 

Although the investment costs for 

conventional wastewater treatment 

systems depend principally on 

wastewater flow rate, the operating 

costs depend on the following 

factors: 

• Sludge disposal and municipal 

sewer fees 

• Wastewater treatment chemical 

costs 

• Operating and maintenance labor 

costs 

• Utility costs (primarily electricity 

to operate pumps and agitators) 

• Overhead costs and depreciation. 

Of these, sludge disposal and 

chemical costs offer the best 

opportunities for reducing operating 

costs since labor and utility 

requirements are usually fixed. The 

cost components of these and other 

operating costs are described below; 

Table 4 shows the assumptions for 

these evaluations. 

Table 4. 

Basis for Economic Evaluations3 

Basis 

Annual operating costs: 

Operating labor 

Supervision 

Maintenance 

General plant overhead 

Depreciation (10-year straight line) 

Taxes and insurance 

Utility charges: 

Electricity 

Cooling water (cooling tower) 

Steam 

Net operating savings ($/yr) 

Net savings after taxes ($/yr) 

Return on investment 

Cash flow ($/yr) 

Payback period (yr) 

$9/h 

$12/h 

6% of investment 

0.58 (operating + supervisory + 

maintenance labor); maintenance labor -

0.37 x (maintenance cost) 

10% of investment 

1% of investment 

$0.08/kWhr 

$0.25/1,000 gal 

$6.00/1,000 Ib 

Operating cost reduction resulting from 

investment minus increase in fixed and 

operating cost for new system 

Net operating savings x 0.54 (assumes 

46% tax rate) 

Net savings after taxes * total installed 

investment 

Net savings after taxes + depreciation 

Total installed investment + cash flow 

*No interest on capital is included in the economic analyses. 

13 

w 
P.13 

Sludge Disposal and Municipal Fees. 

Installation of wastewater treatment 

systems results in the discharge of 

two streams: overflow from the 

clarifier and sludge from the clarifier 

or dewatering equipment. The costs 

associated with these discharges are 

site-specific, and depend on the 

availability of local disposal sites to 

receive the sludge and on municipal 

sewer costs. These costs are 

expected to escalate in the future as 

new regulations are implemented. 

For the overflow from the clarifier, 

typical sewer fees for a major city as 

presented in Table 1 are $1.20/1,000 

gal. Figure 8 shows the direct 

relationship of wastewater flow rates 

and these sewer fees. 

The cost of hauling the sludge to a 

licensed hazardous waste landfill will 

depend on the volume of sludge, the 

distance hauled, and the sludge 

composition. In most areas, the costs 

of sludge disposal are currently in 

the range of $.50 to $1.00/gal, but 

there are cases where disposal costs 

run as high as $2.00/gal. Figure 9 

shows the annual disposal costs for 

each 100 Ib. (45 kg) of solids 

generated daily over a range of 

sludge concentrations and disposal 

costs. For example, if 100 Ib of solids 

are generated daily at a 

concentration of 6% and at a 

disposal cost of $1.00/gal, the annual 

cost for disposal will be $60,000 

(point A on Figure 9). 

The disposal cost savings achievable 

by thickening can also be estimated 

by using Figure 9 to calculate the 

difference between the disposal 

costs at the present concentration 

and at the projected final 

concentration after thickening. For 

example, a plant now disposes of 

100 Ib/d (45.4 kg/d) of dry solids as a 

sludge with a concentration of 6% 

solids. From Figure 9, the disposal 

cost of 100 Ib/d at $1.00/gal would be 

$60,000 per year. If a filter press 

performance test predicts a sludge 

concentration of 25% solids, sludge 

disposal costs would decrease to 

$14,000. The disposal costs saved as 

a result of greater dewatering of the 

sludge (from 6 to 25% solids 

concentration) would then be 

$46,000 ($60,000 minus $14,000). 

50 

40 

(000 ($1 -SEWER FEE 

CO 

o 

1 

OT 

8 20 ANNUAL 

10 

lll l 

0 2 4 6 8 

CLARIFIER OVERFLOW RATE (1.000 gal/hl 

Notes: 

Operating 4800 h/yr. 

Sewer fee = $1.20/1000 gal. 

Water use fee not included. 

I 

10 

Figure 8. Annual Sewer Fee as a Function of Clarifier Overflow Rate 

14 i 2 . 
P.14 

1000 rrf 100 

V) 

o 

o. 

- m 

5 

< 

i io 

z < 

J I I I 11 l l J — I I I J-il 

$2.00/gal sludge 

disposal cost 

.$1.00/gal sludge 

disposal cost 

,$0.60/gal sludge 

disposal cost 

.1 I I u l 

0.1 1.0 10 

CONCENTRATION SOLIDS IN SLUDGE (wt%) 

100 

Rgure 9. Annual Cost for Disposal of Industrial Sludge 

(per 100 Ib dry solids generated per day) 

Filter press hopper with 40% solids filter cake. 

15 

3 
P.15 

600 

500 

000) ($1 

Z ^ 

UJ 5 

i— 

OT 

UJ 

> 

- 300 CAPITAl 

UJ 

___ 200 

< JUSTIF 

100 

0 

— $2.00/gal disposal cost 

^ 

S^ 

f 

_ 1 $1.00/gal disposal cost 

/ ^ ' 

I m^^ 

" 1 f 

I / $0.50/gal disposal cost 

1 1 .____»- ^^^ M 

v/^^ 

I 1 Illl l 

5 10 15 20 25 30 

CONCENTRATION OF DEWATERED SLUDGE (% solids) 

Notes: 

Assumes 100 lb/day dry solids generated. 

Assumes 40% ROI (pretax). 

ROI = (disposal cost savings - fixed cost) 

+ capital investment. 

Fixed cost includes depreciation, taxes. 

insurance and maintenance. Utility 

and labor costs not included. 

In a 2% solution, 100 Ib solids = 600 gal. 

5% = 232 gal. 

10% = 112 gal. 

39% = 32 gal. 

Figure 10. Capital Cost Justification for Sludge Dewatering Equipment 

Mechanical dewatering of metal 

hydroxide sludges can achieve 

solids concentrations in the range of 

15 to 50%. Figure 10 shows the 

capital investment that could be 

justified for dewatering equipment 

to concentrate a sludge containing 

only 2% solids. The cost reduction 

used to calculate this return on 

investment (ROI) did not take into 

account variable operating costs, 

such as utility costs and operating 

labor, which for mechanical 

dewatering devices could be 

significant. These additional costs 

would result in a lower ROI. 

To illustrate the investment 

justification for the mechanical 

dewatering of a dilute sludge, 

assume a plant generating 100 Ib/d 

of dry solids was able to concentrate 

its sludge from 2 to 20% solids with a 

filter press. The annual disposal cost 

would be reduced from $180,000 to 

$16,000 based on a disposal cost of 

$1.00/gal (Figure 9). The maximum 

capital investment justified to 

achieve this saving, using Figure 10, 

is $270,000. Figure 7 shows the cost 

of filter presses as a function of cake 

voiume and feed-processing 

capacity. Using the feed-processing 

capacity, the press feed rate would 

equal: 

1001b solids 

gal sludge 

8.341b 

x Ib sludge y 

0.02 Ib solids 

d _ 75 gal sludge 

_ _ _ 

From Figure 7, the minimum size 

commercial unit could dewater this 

feed rate and would cost $16,000. 

Assuming the total installed cost of 

the system is twice the press cost, or 

$32,000, the ROI would be well in 

excess of the 40% used as a basis for 

Figure 10. 

16 \4 
P.16 

Wastewater Treatment Chemical 

Costs. These chemical costs are 

dependent on the concentration of 

pollutants, the volumetric flow rate 

of the waste stream, and the types of 

chemicals chosen for wastewater 

treatment. Because the addition of 

chemicals involves a chemical 

reaction with the pollutants, the 

types of treatment chemicals 

selected will produce different use 

rates, volumes of sludge for 

disposal, and removal efficiencies. 

Each of these factors affect the 

operating costs and must be 

considered by the plater. 

Common treatment reagents used in 

the electroplating industry are 

sodium bisulfite (NaHS03) for 

chromium reduction, sodium 

hypochlorite (NaOCI) for cyanide 

oxidation, and caustic soda for 

neutralization/ precipitation. These 

chemicals, and the chemical costs 

given in Table 1, were used to 

provide the cost model for 

conventional wastewater treatment 

shown in Figure 11. This model 

enables the user to calculate the 

consumption of treatment chemicals 

(consumption factor) and the 

associated cost (cost factor), based 

on the volumetric flow rate of the 

wastewater being treated and mass 

flow of each pollutant. The sludge 

disposal cost model (shown in 

Figure 12) can be used to determine 

the dry solids generated by 

precipitation of the heavy metals 

contained in the waste stream. 

Figure 12 also indicates the disposal 

cost for each pound of metal 

precipitated, assuming the resulting 

sludge is 20% solids and the disposal 

cost is $1.00/gal. 

Many treatment systems use 

hydrated lime for neutralization 

instead of sodium hydroxide; some 

systems use soda ash. The selection 

of neutralization chemicals is usually 

based on convenience and price. The 

choice of neutralization reagent will 

affect the volume of sludge 

generated, the costs of sludge 

disposal, and the investment cost for 

storage and handling for each 

chemical. A cost comparison of the 

various neutralizing reagents is 

shown in Table 5. Lime requirements 

and chemical and sludge disposal 

costs can be estimated using the 

model in Figure 13, which is similar 

to the models developed in Figures 

11 and 12 for caustic neutralization. 

The cost models in Figures 11,12, 

and 13 can be used to determine the 

incremental costs associated with 

removing each pound of pollutant 

and treating each gallon of 

wastewater entering the system. 

These models can also be used to 

predict the impact of water use 

reduction programs, chemical 

recycle systems, and other 

modifications on the costs of 

operating a wastewater treatment 

system. 

17 
P.17 

Cyanide Waste 

gal/min 

IbCN" 

Ib Metal* 

• 

Oxidation (NaOCI. 

7.5 Ib NaOCI/ 

IbCN $4.50/Ib CN 

Chromium Waste 

gal/min 

IbCr*6 

• 

Reduction (NaHS03, H2S04)

c 

3 Ib NaHSO^b Cr"6 

2 Ib H2S0«/lb Cr*6 

0.3 Ib NaHSO. 1000 gal 

0.2 Ib H2SO4/IOOO gal 

$i.oo/ibcr6 

$.10/1000 gal 

• 

Neutralization (NaOH)d 

1.51b NaOH/1000 gal $.20/1000 gal 

* 

Precipitation (NaOH) 

2.3 Ib NaOH/lb Cr 

2.0 Ib NaOH/lb Metal 

Legend: 

Process Step (Treatment Reagent) 

Consumption 

Factor 

Cost 

Factor 

$.30/lb Cr 

$.26/lb Metal 

* 

Flocculation (Polyelectrolyte) 

0.1 lb/1000 gal $.20/1000 gal 

j * Clarification 

Sludge Storage 

• 

Dewatering 

t 

Disposal 

Wastewater 

Discharge' 

Acid/Alkali Waste 

gal/min 

Ib Metal* 

* 

Neutralization (NaOH)'' 

1.01b NaOH/1000 gal $.13/1000 gal 

* Lb metal expressed as Ib of metal ion 

C 

C 

Chromium reduction using Su2 : 

Reduction <S02, H2SOJ 

2 Ib SOa/lb Cr' 6 

0.2lbSOj/1000gal 

0.2lbHjSO4/1000gal 

$.28/lb Cr*6 

$.07/1000 gal 

Cyanide oxidation using Cl 2 : 

Oxidation (Cl2, NaOH) 

7 Ib Clj/lb CN 

8 Ib NaOH/lb CN $1.67/lbCN 

Lime is also commonly used; a usage 

Figure 11. Consumption and Cost Factors for Wastewater Treatment Chemicals 

18 \h 
P.18 

Cyanide Waste 

(gal/min. Ib Metals) 

Chromium Waste 

(gal/min, Ib Cr) 

Acid/Alkali Waste 

(gal/min, Ib Metals) 

Precipitation (NaOH) 

Ib dry solids generated 

Ib of metal precipitated 

Cr 1.98 

Ni 1.58 

Cu 1.53 

Cd 1.30 

Fe 1.61 

Zn 1.52 

Al 2.89 

Flocculation/ 

Clarification 

I 

Sludge Storage/ 

Dewatering 

Notes: 

Ib metal expressed as Ib of metal ion. 

Disposal cost at $1.00/gal. 

1 20% Solids 

Solid Waste Disposal Cost 

($/1b of metal treated) 

Cr 

Ni 

Cu 

Cd 

Fe 

Zn 

Al 

$1.18 

0.92 

0.90 

0.78 

0.96 

0.92 

1.72 

Figure 12. Generation and Cost Factors for Sludge Disposal 

Table 5. 

Cost Comparison of Common Alkaline Reagents 

Wastewater 

Discharge 

Agent 

Sodium hydroxide 

(caustic) 

Sodium carbonate 

(soda ash) 

High calcium 

hydrate (hydrated lime) 

High calcium 

lime (quicklime) 

Chemical 

formula 

NaOH 

Na_C03 

Ca<OH>2 

CaO 

Price 

($/ton) 

$260 (98% 

NaOH 

equivalent) 

$120 (58% 

NasCOj) 

$50 (96% 

pure) 

$40(96% 

pure) 

lb/lb 

HjSO< 

neutralized 

0.826 

1.90 

0.788 

0.595 

Relative 

cost 

9.0 

9.58 

1.65 

1.00 

Process equipment required 

and sequence for use 

of reagents 

Caustic storage — Neutralizer 

Soda ash storage —• 

Slurry tank—* Neutralizer 

Hydrated lime storage —» 

Slurry tank — Neutralizer 

Quicklime storage -» Slaking -• 

Slurry Tank-» Neutralizer 

19 

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