Återvinning av vattenverksslam

Summary
When producing drinking water from surface water sources, the most common
treatment method is chemical coagulation and precipitation. In the coagulation
step, an aluminium salt is often added in order to cause impurities such as humic
acids and particles to coagulate and precipitate from the raw water. The treatment
method produces sludge which in addition to the impurities from the raw water
also consists of added precipitation chemical. The problem of handling the sludge
always has to be solved at the waterworks. Sludge causes sometimes a negative
environmental impact.
In order to solve the problem with aluminium­containing sludge, a new
process called “the REAL­process” has been tested during a year in pilot scale at
the Hässlö waterworks in Västerås, Sweden. The construction of the pilot plant
and the work during the test period has been carried out as a VA­Forsk project,
where Mälarenergi AB, Sydvatten AB, Karlskrona municipality, Feralco AB,
University of Lund, Mercatus Engineering AB and Sweco Viak AB participated.
The REAL­process is a recycling process with four steps for aluminium recovery
from waterworks sludge. In the first step, aluminium hydroxide is hydrolysed by
addition of sulphuric acid to the sludge. In the second stage, ultrafiltration (UF)
is utilised for separating all suspended material and macromolecules from the
dissolved aluminium ions. The sludge volume is reduced by the UF approximately
20 times and the sludge contains a very small amount of aluminium. Permeate
from the UF contains the vast part of the aluminium ions. It is led to the third
process step, a nanofilter (NF), where the concentration of the aluminium is
increased. In the fourth step, the aluminium is precipitated by addition of
potassium sulphate, causing the aluminium to form potassium aluminium
sulphate dodekahydrate crystals (alum). The pure salt can be reused as coagulation
chemical in the waterworks. The recovery is approximately 90 %.
The REAL­process is not to be seen purely from an economical point of view,
but also from an ecological, since a solid waste (waterworks sludge) is recycled
as new chemical. Also from an economical point of view, the REAL­process is
keeping up well.