1. Title
Responsible dosing of iron(II)sulphate in a drinking water source; in control of coagulation.
2. Company information
Dunea supplies drinking water to 1.2 million people and welcomes each year over a million
visitors to the dunes between Katwijk and Monster. With approximately 520 employees we
supply around 73 billion litres of drinking water each year to our customers in the western part of
South-Holland. This results in a revenue of 140 – 150 mln euros. Besides supplying drinking
water we conserve around 2200 hectares of dune area.
Every year Dunea invests around 800k€ in R&D activities and partnerships. Most of this is
invested in a sector-wide research programme focussed on drinking water specific topics.
3. Problem
For the production of drinking water Dunea uses artificial dune water (using MAR, managed
aquifer recharge). Surface water is transported to, and infiltrated in the dunes to create artificial
groundwater. The source of this water is the "Afgedamde Maas", a side branch of the river Meuse
(See also Fig. 1 & 2 below).
The Afgedamde Maas runs through an active agricultural area called the Bommelerwaard from
which excess surface water is pumped into the river. Together with the supply from the river
Meuse this causes an increase in the amount of nutrients in the water, increasing the risk of algae
bloom due to eutrophication. This effect occurs in many places all over the world. There are many
different solutions available to limit the amount of phosphate, dosing iron(II)sulphate is one. At
this moment Dunea primarily focuses on this technique, we are however also open to new
solutions to coagulate phosphate and prevent algae bloom.
In order to prevent the growth of algae (especially blue algae) due to the limited flow in the
branch, Dunea currently doses dissolved iron(II)sulphate which coagulates phosphate and limits
the food source for algae. Iron(II)sulphate is dosed at the bottom of the branch, in the fairway, via
a HDPE pipe with nozzles. Around this iron dosing pipe, a second HDPE pipe is mounted with
holes situated at the iron dosing nozzles. Air is dosed via these holes in the second pipe for
mixing purposes (iron(II)sulphate with river water) and for compensating oxygen consumption
for the oxidation of Fe(II) in Fe(III). In chapter 6 of the attached essay about cyanotoxins in
drinking water treatment, more detailed information can be found about the coagulation.
The current dosing system is at the end of its lifetime and is unable to fulfil future requirements
like intermittent dosing. Several challenges exist when upgrading the present dosing system and
optimizing the dosage :
If the iron(II)sulphate dosing pumps are shut off, in case of a failure or for maintenance reasons,
iron(II) reacts in the nozzles till iron(III) and precipitates as iron hydroxide and iron phosphate.
On yearly base, divers have to clean the nozzles for this reason. In between cleaning the
performance of the installation worsens.
As a dosing optimization, future dosing strategy will be intermittent. If the nett flow is in the
direction of the Meuse river, dosing will be stopped, to prevent loss of dosed iron into the Meuse
river. It is of utmost importance to prevent reaction of iron(II) in the dosing system if the dosing is
stopped on regular basis (daily).
The dosage is currently not evenly distributed in the water phase, there is overdosing in the
fairway and under dosing at the edges of the river. This is also visible in fig. 2, the bubbles are
only in the fairway. From environmental perspective, locally low pH, this is undesirable. Next to
it, the coagulation of phosphate could be influenced. There is however more flow in the centre of
the river, this has to be considered (modelled?) .
Currently we use air for mixing purposes, does this create an optimal mixture? Optimization of
mixing can limit the necessary air input. The optimum situation is only air input for
compensation of the oxidation of iron(II) to iron(III), on condition that the mixing of ironsulphate
with water is secured.
Dunea does not have the expertise to design a proper dosing system for dissolved
iron(II)sulphate, without the occurrence of unwanted coagulation in the dosing system itself. For
uninterrupted operations and optimization reasons, this is of utmost importance.
In the current system, Dunea dissolves iron(II)sulphate to be able to dose it using the described
system. We would be happy to work on more durable solutions for example based on dosing
iron(II)sulphate as crystals.
Boundary conditions:
The installation must be able to comply with all applicable regulations and our permit(s). It must
fit the current site. The solution must take into account environmental friendliness. Preferably be
modular (easy scale-up and –down).
The solution must be based on dosage of iron(II)sulphate as stipulated in our permit. A recent
study has shown that this is the best available technique (report and permit are available as
input).
Dosing site
Fig. 1. Overview River Meuse, Afgedamde Maas and Dosing Site.
Fig. 2. Dosing site in detail, Chemical Building on the left and bubble-track in the centre of
the river.