A Safe Replacement for DTC

To meet regulatory limits for discharging process wastewater containing metals, industry has traditionally used commodity chemicals such as sodium hydroxide, lime, iron chemistries and magnesium hydroxide to precipitate metals for removal prior to discharge. However, these chemicals are not capable of precipitating chelated or complexed ionic metals often present in process wastewater.

To achieve the limits established by federal, state and local regulatory agencies, industry turned to sodium dimethyldithiocarbamate (DTC), which has become the most common co-precipitant chemistry for precipitating chelated/complexed metals in process wastewater. In most cases, DTC helped solve the problem of meeting lower numeric discharge limits.


However, to achieve the lower limits with DTC-based products, a tradeoff occurred. DTC is a very toxic chemistry that is also being used as an industrial biocide and as an agricultural insecticide. Last year a plating plant in Indiana discharged treated wastewater containing high amounts of DTC that impacted the local publicly owned treatment works (POTW) and, in turn, the river receiving the POTW discharge. The result was 117 tons of dead fish along a 50-mile stretch of the White River.


DTC is a very toxic chemistry that is also being used as an industrial biocide and as an agricultural insecticide.

Recently, the federal government proposed new regulations that will set limits for metals in wastewater discharges that are approximately 50 to 80 percent lower than existing standards. In addition, limits are being proposed for several pollutants that have not been previously regulated, such as manganese, molybdenum, sulfides and tin.

Table 1. Comparison of Existing Pretreatment Standards with

Proposed MP&M Limits milligrams per liter (mg/l)

413

433

Proposed

MP&M

Daily

4-Day Avg.

Daily

Mon. Avg.

Daily

Mon. Avg.

Cadmium

1.20

0.70

0.69

0.26

0.21

0.09

Chromium

7.00

4.00

2.77

1.71

1.30

0.55

Copper

4.50

2.70

3.38

2.07

1.30

.057

Cyanide (T)

1.90

1.00

1.20

0.65

0.21

0.13

Cyanide (A)

0.86

0.32

0.14

0.07

Lead

0.60

0.40

0.69

0.43

0.12

0.09

Manganese

0.25

0.10

Molybdenum

0.79

0.49

Nickel

4.20

2.60

3.95

2.38

1.50

0.64

Silver

1.20

0.70

0.43

0.24

0.15

0.06

Sulfides

31.00

13.00

Tin

1.80

1.40

Zinc

4.20

2.60

2.61

1.48

0.35

0.17

TTO*

2.13

2.13

Total Metals

10.50

6.80

TOC**

78.00

59.00

TOP***

9.00

4.30

* Total Toxic Organics

** Total Organic Carbon

*** Total Organic Parameter

The DTC Replacement

Thio-Red, a patented metal precipitant, is being used by metal plating plants with great success to safely precipitate mixed chelated metals from discharge waters. In addition to being much less toxic than DTC, the new precipitant offers these additional advantages:

  1. Generates up to 50 percent less sludge than DTC, Lime or Iron chemistries;
  2. Achieves the proposed mixed metals discharge levels in most wastewater;
  3. Requires lower dosing of flocculent chemistry for liquid/solid separation;
  4. Usage is typically 50 percent lower than DTC for comparable, or better, mixed metals removal;
  5. Properly treated waters pass toxicity tests for National Pollutant Discharge Elimination System (NPDES) discharges; and
  6. It is compatible for microfiltration applications.

The following case studies, performed in 2000, are examples of how the use of the new precipitant product resulted in cost reductions through chemistry savings and decreases in sludge volumes while, at the same time, achieving the new proposed lower level wastewater limits.

Case Studies

Early in 2000, a New England decorative/electronic plating shop was using DTC to precipitate mixed metals in their process wastewater stream. A water conservation program was initiated which changed the profile of the untreated wastewater. This change necessitated a significant increase in the DTC dosage in order to achieve the current metals discharge levels required by the local POTW. However, excess DTC will allow for the generation of excess carbon disulfide, which is a permitted substance on their discharge permit.

A Thio-Red product (TR-50) was tested and put on line to replace the DTC precipitant. Tables 2 and 3 are the chemical and sludge data collected from the facility over the past year:

Chemical

lbs. used / Day

Cost / Day

Cost / Year

DTC

108

$ 78

$ 20,592

TR-50

35

$ 44

$ 11,616

Table 2

Program

Tons sludge generated

/ Month

Hauling cost

/ Ton

Cost / Month

Cost / Year

DTC

8.75

$ 285

$ 2,493

$ 29,925

TR-50

4.50

$ 285

$ 1,282

$ 15,390

Table 3

As you will note from the above data, the annual combined chemical and sludge removal cost was reduced from $50,517 to $27,006, a $23,511/ year savings. In addition to the cost savings, the use of the new precipitant product resulted in the following benefits for the company's wastewater treatment operation:

  1. Better and consistent overall metal reduction;
  2. Filter cloths no longer clog as they did with the DTC program;
  3. The clarity of the discharge water is greatly improved; and
  4. Improved sludge dewatering, which results in much dryer sludge cakes.

In the second case study, a large plating facility in Connecticut routinely treats 100,000 gallons per day of process wastewater generated by an alkaline washing and mixed metals plating operation. In the past, treatment of the process wastewater consisted of using large volumes of inorganic chemistries, then the treated water was discharged directly into a river.


Recently, the federal government proposed new regulations that will set limits for metals in wastewater discharges that are approximately 50 to 80 percent lower than existing standards.

Because of the variety of wastewaters from the plating job shop, the plant experienced problems with metal excersions in the discharge water as well as occasionally failing the required toxicity testing.

A Thio-Red program was bench tested for about one year before deciding on a system trial. The system trial was put on line one day before a required metals and toxicity sampling day. The change over to the new precipitant product went very well and tests showed that both mixed metals and toxicity levels passed the existing stringent discharge limits, as well as the U.S. Environmental Protection Agency's (EPA's) new proposed discharge limits.

Tables 4 and 5 show the data collected to compare chemistry costs, as well as the sludge volume and subsequent costs.

Old Chemistry Program

Chemical

Lbs. / Day Used

Cost / Day

Cost / Year

FeSO4

700

$ 113

$ 29,832

Lime

200

80

21,120

MgOH

200

80

21,120

Al Coag

-

225

59,400

Polymer

-

45

11,880

TOTAL

$ 143,352

New Chemistry Program

Chemical

Lbs. / Day Used

Cost / Day

Cost / Year

TR-50

330

$ 214

$ 56,496

AQ-202

164

156

41,184

FeSO4

100

16

4,224

Polymer

-

36

9,504

TOTAL

$ 111,408

Table 4

Sludge Generation Old Chemistry Program

Hauling Cost / Ton

Tons Generated / Month

Cost / Month

Cost / Year

$ 100/ton + $1,000

28

$ 3,800

$ 45,600

Sludge Generation TR-50 Chemistry Program

Hauling Cost / Ton

Tons Generated / Month

Cost / Month

Cost / Year

$ 100/ton + $1,000

13

$ 2,300

$ 27,600

Table 5

In the old chemistry program the total cost for chemicals and sludge hauling was $188,952.

With the new chemistry program the total cost for chemicals and sludge hauling was $139,008.

The total annual savings was $49,944.

The reduction in chemical and sludge removal costs amounts to estimated annual savings of approximately $50,000. At the same time, lower metals numbers and toxicity levels are achieved as required for direct discharge to a river.

Because the wastewater treatment operators are no longer required to handle tons of inorganic chemistries, and spend considerably less time operating and cleaning the filter press, they have time to do other maintenance tasks which under the old chemistry program quite often went undone because of lack of time.




This article originally appeared in the September 2001 issue of Water & Wastewater Products, Volume 1, Number 2, page 40.

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