CH#824 - Steel Production - Activated Sludge

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Courtesy of BIO-SYSTEMS International

Background
The treatment plant at this Steel Mill treats approximately 0.3mgd waste from the coke and steel operations. Over the past year the treatment works improved performance and consistently produced a good quality effluent with respect to phenol, suspended solids, ammonia and cyanide. An improvement in performance was required to meet future discharge limits.

To achieve this level of treatment, the industry operates an activated sludge plant with two aeration basins in series and two secondary clarifiers in series. The activated sludge system comprises an above ground concrete tank with fixed high speed aerators operated with a high solids level to achieve single stage nitrification and twin secondary clarifiers with scraper mechanisms.

The application of Bio-Systems products was considered for the following reasons:

Ammonia was consistently below city discharge requirements with occasional peaks to 30 mg/l, however, more consistent nitrification is required to meet the future standard. Bio-Systems products were selected to lower the presence of inhibitory compounds thereby achieving higher rates of ammonia removal.

Cyanide The future standard required a lower level of cyanide in the final effluent. Improvement in the effluent cyanide concentration is dependent upon many factors, however, Bio-Systems products are expected to achieve this due to:

  • Enhanced cyanide degradation
  • Better entrapment of cyanide complexes
  • Lower ammonia concentrations, resulting in use of the nitrogen component of the cyanide as a nutrient.

In order to fully evaluate the effect of Bio-Systems products on the treatment plant a trial was commenced August 17, 1887.

Discussion
The data for July 1987 is the control data and the period of Sept 3, 1987 thru October 19,1987 is the treated period data. The influent concentrations for the various parameters were very similar during these two periods and the plant operating conditions were similar.

Nitrification
Nitrification is the process whereby ammonia is converted to nitrate then nitrite through bacterial action. The controlling process rate is usually the ammonia to nitrate step.

Nitrification occurs in the aeration basin under aerobic conditions and requires good operating conditions. Nitrifying bacteria are inhibited by low concentrations of toxic chemicals even on an intermittent basis, for instance at ten day intervals. Nitrifiers are also very slow to grow and therefore a long sludge age is nearly always needed.

From earlier reviews of the historical operating data we concluded that the plant had not nitrified on a regular basis previously. This review of the data indicated that the effluent ammonia concentrations usually ranged 4 to 30 mg/l. If nitrification occurs, then effluent concentrations are normally less than 5mg/l and frequently less than 1 mg/l.

Conclusion
In conclusion, the data and observations indicate the treatment plant did not historically achieve nitrification on a regular basis.

Bio-Systems reduces the frequency and levels of chemical spikes thereby enabling the nitrifier organisms to function at a lower inhibition level. During the addition of Bio-Systems products to the treatment plant, the degree of nitrification increased significantly due to the reduced presence of inhibitory compounds and enhanced rate of nitrifier growth encouraged by the Bio-Systems product.

The performance of the plant during the addition of Bio-Systems is better than previously experienced with respect to nitrification. If the ultimate requirement for this plant is to nitrify in order to maintain an effluent meeting the new standard, then we recommend continued addition of Bio-Systems products.

Cyanide Removal
The impact of Bio-Systems reduced the peaks of cyanide breaking to the effluent. At fifty percentile level there was very little difference between the July data and the Bio-Systems data. However, at the 90 percentile the data was:

  • Without Bio-Systems 9.2 mg/l
  • With Bio-Systems 6.3 mg/l

The data for the last fifteen days of the trial the mean cycle cyanide concentration dropped to 4.75 mg/l compared to 5.99 mg/l. The traveling five day mean concentration of cyanide in the effluent was on a steady decline throughout the application of Bio-Systems and then increased after Bio-Systems was removed from the treatment plant.

Conclusions
The addition of Bio-Systems to the treatment plant at the steel mill achieved the following benefits:

  • Enhanced Nitrification - resulting in lower effluent ammonia concentrations
  • Enhanced Cyanide Degradation - a thirty one percent improvement in the ninety percentile cyanide concentration.

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