A Safe Replacement for DTC

Finally - A Safe Replacement for DTC
An alternative for precipitating mixed cheleted metals from process wastewater comes clean.
      By Dean M. Schmelter,
Water Specialists Environmental Technologies, Sanford FL

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 allegedly discharged treated wastewater containing high amounts of DTC that impacted the local 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.

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

Comparison of Existing Pretreatment Standards with Proposed MP&M Limits (mg/l)
Existing Standard 40 CRF 413 (mg/l) Existing Standard 40 CRF 433 (mg/l) Proposed MP&M Discharged Limits
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, Thio-Red offers these additional advantages:

  • Generates up to 50% less sludge than DTC, Lime or Iron chemistries
  • Achieves the proposed mixed metals discharge levels in most wastewaters
  • Requires lower dosing of flocculent chemistry for liquid/solid separation
  • Usage is typically 50% lower than DTC for comparable, or better, mixed metals removal
  • Properly treated waters pass toxicity tests for NPDES discharges
  • Compatible for microfiltration applications

Case Studies in the year 2000
The following Case Studies, that were performed in 2000, are examples of how the use of a THIO-RED 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 STUDY #1
Early in 2000 a New England decorative/electronic plating shop was using DTC to precipitate mixed metals in their process waste 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. The following is 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
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

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 THIO-RED resulted in the following benefits for their wastewater treatment operation:

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

A large plating facility in Connecticut treats 100,000 gallons/day of process wastewater generated by an alkaline washing and mixed metals plating operation. Treatment of the process wastewater consisted of using large volumes of inorganic chemistries, then the treated water is discharged directly into a river.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 1-year before deciding on a pilot trial. The system trial was put on line 1 day before a required metals and bio-monitoring day. The change over to the Thio-Red program went very well and tests showed that both mixed metals and toxicity levels passed the existing stringent discharge limits, as well as the new EPA proposed discharge limits.

The following is the data collected to compare chemistry costs as well as the sludge volume and subsequent costs:

Old Chemistry Program:

Chemical lbs. used / Day 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. used / Day 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

Sludge Generation Old Program:

Hauling Cost / Ton Tons Generated / Month Cost / Month Cost / Year
$ 100/ton + $1,000 28 $ 3,800 $ 45,600

Sludge Generation New Program:

Hauling Cost / Ton Tons Generated / Month Cost / Month Cost / Year
$ 100/ton + $1,000 13 $ 2,300 $ 27,600

Old Chemistry Program: total cost for chemicals & sludge hauling = $ 188,952
New Chemistry Program: total cost for chemicals & sludge hauling = $ 139,008
The reduction in chemical and sludge removal costs amounts to an 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 waste treatment operators no longer are required to handle tons of inorganic chemistries, and spend considerably less time operating & 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.If you would like samples or further information please contact us.