Pre-filtering saves 50% investment in sewage plant

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Source: Biowater Technology AS

Arendal Municipality: Sewage plant doubles its capacity without expanding buildings. New pre-filtering solution halves investments, brings better sludge, reduces operational costs and shrinks carbon footprint.

The Saulekilen Sewage Plant in Arendal, south of Norway, increases its capacity from 45000 p.e. to 80000 p.e. with a new process design. By using a first filter stage combined with compact biofilters, the new plant fits inside the old one. This reduces investments by NOK 25 mill. (USD 4 mill.), which is approximately half the cost of a conventional treatment plant.

Filters at entrance

The pre-filtering arrangement takes out particles. This simple but effective treatment cuts the downstream loads, reduces size and running costs of the sewage plant. The sludge from the filters will also bring higher output from the in-house gas production.
The pre-filtering step consists of Salsnes Filters, a Norwegian system which harvest 35% of the total sludge volume. This is done before the wastewater enters the downstream biological treatment process. 
- This has some advantages, states technical director Knut Berg-Larsen at Arendal Municipality.

More biogas

The sludge from the Salsnes Filters is pumped to storage, stabilized and goes further to methane reactors. Chemicals such as flocculants are not added to the Salsnes Filters; therefore, the “virgin sludge” generates the highest output of biogas.

- The methane we shall produce will be used for internal energy production, reducing such costs and contributing to a smaller carbon footprint, according to Berg-Larsen.
The higher energy output from pre-filtered sludge compared to secondary sludge digested from a conventional secondary treatment plant, provides a good return on the investment.

More phosphorous

Another advantage the pre-filtered sludge provides is that it will be better for agriculture.  
- This ”virgin sludge” will have a higher nutritional value for plants, because it has no phosphorous binding flocculants added. Most flocculants inhibit phosphorous release and therefore reduces its availability to plants, according to Berg-Larsen.
As phosphorous is a limited global resource, steadily increasing in price, the residuals from this treatment plant (i.e. the “virgin sludge”) will command a higher value than residuals from a conventional plant.
- We expect it will be simple to find a market for this sludge, according to Berg-Larsen.

Smaller plant

With filtering as the first step, the total load on downstream processes is less. The filters are placed on top of biological filters, reducing need for space, making all the former basin volumes available for new bioreactors and clarification of biological solids by dissolved air flotation.  
In sum the filtering ensures a more compact system, to be fitted inside the existing buildings.
- With this design we can build the treatment plant NOK 20-25 mill. cheaper than with a traditional layout. We have chosen this design because it’s the only way we can fit the new plant inside our existing buildings. The savings are considerable on different levels, and we expect the plant to be less expensive to operate, according to Berg-Larsen.

Shorter construction time

The first step in building the new plant has been to install the new filters with the existing plant running. This initiative has made no other special installations necessary during construction, hence made the re-furbishing the new plant more rapid. This has been done with consent from the County Environment office.     
- The approval from the County Environment office makes it possible for us to pursue a swift construction plan. The installation of the filters in the first stage makes is possible to minimize both construction cost and final discharge pollutants, according to Berg-Larsen.

A new step

Arendal Municipality recognized the new way of filtering early in the project.  
- Even if we were to build anew we would have chosen this process. The alternatives that we know of would come out more costly, and this also goes for operational costs, according to Berg-Larsen.
Biowater Technology of Tønsberg is also the main contractor for all construction work at Saulekilen RA in Arendal including all process equipment (Salsnes Filter, CMFF®, and DAF).
- The fact that Arendal wants a sustainable solution was a perfect match for us, since minimum carbon footprint is the focus of our technology. We regard Arendal’s choice as a proof of confidence, according to Jon Gregar Siljudalen, Chief Process Officer of Biowater.

The plant will be ready for operation by April 2015, and the total project cost is estimated to be NOK 28 million.

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