Pollution Prevention and Control Technologies for Plating Operations

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Section 6 - Wastewater Treatment

6.2 CONVENTIONAL TREATMENT TECHNOLOGIES

6.2.7 Sludge Generation and Disposal Data from the Users Survey

A summary of sludge generation data from the Users Survey is presented in Exhibit 6-15. Column 1 indicates the plating shop code. Column 2 indicates the type of sludge that is generated (e.g., hydroxide, sulfide). In all cases, shops either generated a hydroxide treatment sludge or no sludge (zero discharge shops and shops that do not plate regulated metals). Examples of the latter case are sulfuric acid anodizing shops that simply adjust the pH of the wastewater and discharge it (e.g., PS 001 and PS 037). The average sludge generation rate for all respondents that reported a sludge quantity is 158,272 lb/yr. Column 4 shows the percent solids of the sludge. The average percent solids for all shops was 54%. Columns 5 and 6 show the unit cost and total annual cost of sludge disposal or recovery for each shop (includes transportation, stabilization, disposal/processing and disposal tax). Columns 7 and 8 show the disposal/recovery site name and the distance between the shop and the site. The average and median distances to disposal/recovery sites are 457 and 300 miles, respectively. On the average, recovery sites are further away from shops than disposal sites. For disposal sites alone, the average and median distance to the sites are 330 and 200 miles, respectively. Additional details on recovery sites are presented in Section 7.

The average unit cost for sludge disposal was $0.53/lb and the median cost was $0.25/lb. Unit disposal costs for shops varied, depending mostly on the quantity of sludge generated. The following range of average unit costs were given for various annual sludge generation rates:

  lbs/Yr of Sludge      Unit Cost, $/lb
  <10,000                      $1.34
  10,001 to 50,000             $0.51
  50,001 to 100,000            $0.43
  100,001 to 250,000           $0.25
  250,001 to 500,000           $0.22
  500,001 to 1,000,000         $0.05
      

Data showing the chemical constituents of sludges generated by some survey respondents are presented in Section 7 (see Exhibit 7-5).

The following methodology can be used to estimate the sludge generation rate for a particular plating shop. It is based on a model presented in reference 392 that was revised, based on the chemical constituent data collected during the Users Survey (see Exhibit 7-5).

Sludge Generation:

(1) Calculate lbs/yr of dry sludge solids:

First, for each metal in the wastewater, multiply the lbs/yr of metal precipitated (this value can be assumed to be equal to the mass of metal in the influent) by its ìsolids generation factor,î and sum.

            Metal  Solids Generation Factor
             Cr            1.98
             Ni            1.58
             Cu            1.53
             Cd            1.30
             Fe            1.61
             Zn            1.52
             Al            2.89
             Other*        1.77 (avg.)
      

*see Exhibit 7-5 in Section 7 for identification of ìotherî metals contained in plating sludges.

(1a) If the sludge will be dewatered above 50% solids, multiple by:

[(0.029 x % Solids*) - 0.41]

(1b) The ìsolids generation factorsî are based on the use of caustic as an alkali reagent. If lime will be used instead of caustic, multiply by 1.13.

(2) Lbs/yr of wet sludge = 100 (lbs dry solids/% Solids*)

*express as a whole number, e.g., 50% = 50

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