White bio ash – ash production and concrete trial casting
The reddish colour of the bio ash concrete manufactured using ashes from AWS WWTP has been identified as a significant barrier, when the concrete is to be used for visible materials or constructions. The reddish colour is most probably due to the use of iron salts for the precipitation of phosphorus, and one possible solution to the problem is to substitute the iron salts by aluminium salts at the WWTP. However, these aluminium salts are more expensive than iron salts.
The task includes the following activities:
a) Collection of chemical analytical data for bio ashes from different WWTP’s in Europe with particular focus on iron content.
b) A test production of concrete using a bio ash incinerated at AWS WWTP. The bio ash is the result of incineration of dewatered digested sludge from a WWTP where aluminium salts have been used for the precipitation of phosphorus. If the colour of the cast concrete is not acceptable, there is no incentive to continue with further studies on the substitution idea.
AWS will act as task coordinator and conduct activity a).
In activity b) the production of Al-rich dewatered sludge will be conducted by LYNIS at the Damhusaaen WWTP by replacement of the phosphorous precipitant over a period of about three months. On this basis, AWS will incinerate a separate batch of sludge during about 2 days period, thus producing white bio ash. A concrete specimen cast will be produced by UNICON and DTI will on the white bio ash perform test according to EN 450-1 5.2 (chemical properties) and 5.3 (physical properties) and evaluate the results.
Progress indicators – 1) Aluminium salt precipitation of phosphorus initiated, 2) Incineration of Al-rich sludge completed, 3) Concrete specimens cast using the Al-rich bio ash.
1) An overview of the iron and aluminium content in various European bio ashes
2) Verification of whether or not the substitution of iron with aluminium for the phosphorus removal results in bio ashes that are colour neutral when used in bio ash concrete
3) Indications about the correlation between iron content of bio ash and the colour of the bio ash concrete.
a) The data collection will depend on the data availability from the literature and / from direct contacts to other European WWTPs with sludge incineration. Contacts will be taken through existing networks.
b) The use of Al-based precipitant has earlier been conducted at the Damhusaaen WWTP so new difficulties are not to be expected. The incineration of a separate batch of dewatered Al-rich sludge is new and could possibly require special attention for tuning the incinerator plant operation.