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Retrofit of the WWTP Walvis Bay ? Enhancement of nitrification rate up to more than 90%

The wastewater treatment plant in Walvis Bay, Namibia was supposed to be modified

It had three existing trickling filters filled with stone media but typical problems occurred such as ponding and clogging. Furthermore, the customer was not satisfied with the performance of the plant. The decision was made to upgrade two of the three filters by using structured packings as fill media ? for the first time in Namibia. The third filter was decommissioned.

The Task

ENEXIO was assigned to upgrade the existing trickling filters in order to achieve a higher performance for COD and ammonia reduction. ENEXIO supported the customer with the new design of the tanks, process engineering and supervision during assembly on site.

The Expertise

The proposal of ENEXIO Water Technologies included BIOdek polypropylene (PP) media. With up to 150 sqm per cu/m, they provide a large effective surface area for microorganisms to grow. At the same time they effectively prevent clogging and ponding.

Additionally PP packings allowed building higher filter beds, thus providing increased performance per footprint. At full operation, stage one, the larger 45-metre diameter trickling filter equipped with a 3.6-metre high packing bed, will remove COD and BOD. Nitrification will take place in stage two in the 30-metre diameter filter with a packing bed height of 1.8 metre.

Above-mentioned measures lead to an excellent performance: With a flow of 5500 cu/m per day, the first trickling filter reduces the incoming COD of 950 mg/l to 240 mg/l and ammonia from 51 mg/l to 4 mg/l. This equals a nitrification of more than 90%.

Finally saving in energy cost could be achieved. The energy consumption to pump the water up to the distributor is 624 kWh per day leading to 0,16 kWh/kg-COD or 0,11 kWh/m3 treated water (including nitrification, excluding pre and post treatment). Energy requirement of the trickling filter is only 10-20% compared to an activated sludge plant, which is a major cost advantage.

The Advantages
-Nitrification rate more than 90% 
-Low energy consumption means low operation costs 
-Savings in logistic costs due to welding of the packings on site 
-Considerable increase of hydraulic and biological performance
-Tanks, main support structure and distributors were designed to meet the process requirements

The Results

The ammonia level instantly dropped to an average of 4.0 mg/l. This means: The ENEXIO solution with a nitrification rate of more than 90% was much better than expected.