Dutch research project to explore chemically enhanced pre-treatment of wastewater
A Dutch research project aims at identifying new, more sustainable wastewater treatment scenarios based on physical-chemical pre-treatment.
By Arjen F. van Nieuwenhuijzen, Adriaan R. Mels and Patty Gerhardt-Meilink
May 23, 2001—A Dutch research project aims at identifying new, more sustainable wastewater treatment scenarios based on physical-chemical pre-treatment. It also evaluated the alternatives on environmental and cost criteria.
This article presents some treatment scenarios based on physical-chemical pre-treatment, which have been identified within the research project. The scenarios have been evaluated by calculating the environmental interventions and the market values.
One of the most promising treatment scenarios from the evaluation was a combination of an advanced pre-treatment (settling or flotation with a combination dosage of inorganic coagulants and polymer flocculants) for particle and phosphorus removal, and a biological fixed bed biofilm system combined with a physical-chemical nitrogen removal technique. The best-known application of this type of treatment plant is situated near Oslo, Norway: the well-studied and documented VEAS sewage works.
To get more practical information about physical-chemical pre-treatment, a study tour was made to Sweden and Norway. The VEAS plant and the KREPRO sludge handling pilot plant at the Helsingborg sewage treatment plant were some of the highlights.
Finally, the research project shows that wastewater treatment scenarios based on physical-chemical pre-treatment can be applied in a more energy efficient way and that plants can be designed to be much smaller than commonly used treatment systems.
Dutch research programme initiated In The Netherlands a research programme was conducted with the objective to develop new, more sustainable wastewater treatment scenarios, based on physical-chemical pre-treatment. The research was initiated and funded by STOWA, the Dutch Foundation for Applied Research on Water Management and was carried out by the Delft University of Technology and the Wageningen University.
Physical-chemical pre-treatment is based on the separation of suspended and colloidal particles from wastewater in a first process stage. Advanced particle removal is important in wastewater treatment, since a major part of the influent COD consists of particulate material. Measurements at several Dutch wastewater treatment plants showed that 60% to 85% of COD is related to particles bigger than 0.45 �m and many pollutants are incorporated into or adsorbed onto particulate material (e.g. phosphorus, heavy metals, organic micro-pollutants and pathogens).
As a result of physical-chemical pre-treatment, the post-treatment will be less loaded with pollutants, so the complete treatment plant may be designed more energy efficiently and compactly [1, 2].
Within the first theoretical phase of the research programme twenty-two complete treatment scenarios based on physical-chemical pre-treatment were identified and evaluated by means of environmental interventions based on sustainability criteria . In addition to these criteria, cost calculations were carried out to compare the identified treatment scenarios [4, 5].
For the design and evaluation of the wastewater treatment scenarios, a model entitled 'Design and Evaluation Model for wAstewater treatment Scenarios' (DEMAS) was developed. The model contains the necessary technical information (design parameters, energy consumption, treatment efficiencies, unit costs, etc.) concerning the various unit processes.
In DEMAS, the unit processes are combined to form complete wastewater treatment scenarios. The system boundaries include all on-site water and sludge treatment processes. Within DEMAS, the identified scenarios are combined with a sludge treatment. This standard sludge handling procedure consists of (gravitational) thickening, followed by sludge digestion. After digestion, the final sludge is transported to a central dewatering and incineration plant.
As a reference scenario, the most commonly applied Dutch 100,000 p.e. wastewater treatment system was used, consisting of simple pre-settling (without chemical addition) followed by a low load activated sludge system for COD, Ptot and Ntot removal and a post-settling tank.
The environmental interventions and the market values can be determined per scenario by aggregation and the treatment scenarios can be compared.
The general conclusions of the calculations are that through advanced particle removal by physical-chemical pre-treatment, wastewater treatment systems can be made more energy efficient and can be designed to be much more compact. Treatment scenarios, which are based on physical-chemical pre-treatment, do not have to be more expensive than commonly used treatment systems, when they are combined with compact biological post-treatment systems.
Treatment systems systems such as those with chemical enhanced flotation respectively settling as pre-treatment and fixed bed low load biofilm systems are especially interesting alternatives. These treatment systems use less energy, are very compact and can compete financially with commonly used sewage treatment plants.
Due to physical-chemical pre-treatment with metal salts, the final sludge production after thickening, digestion and dewatering will increase. This is mainly due to an increased production of inorganic (chemical) sludge. This causes higher sludge handling costs, since the final sludge has to be incinerated and safely dumped (as chemical waste), due to the strict Dutch regulations regarding sludge use on farmland.
During the research, ammonium-nitrogen was identified to be the most difficult component to treat. A decrease in the BOD/N ratio by the removal of BOD during pre-treatment can cause problems with denitrification in the post-treatment. A physical-chemical ammonium treatment technique such as ion exchange could solve these problems.
Continued research in physical-chemical treatment Since the study tour the research activities have continued. The following research items have been investigated in more detail:
-- Characterisation of suspended matter in wastewater. Wastewater influent was fractionated and pollutants were characterised over particle size distributions. Next to biological characterisation of wastewater, this physical-chemical wastewater characterisation adds new information about performances of wastewater treatment plants and offers a tool to model and design physical-chemical treatment systems more adequate.
-- Organic polymers - In a second research phase the possibilities of replacing inorganic metal salts with organic polymer products for coagulation/flocculation of particles were studied. Using jar tests and pilot plants, several different polymer products were tested on raw wastewater. Experiments were conducted on settling characteristics of the formed flocs, and removal efficiencies for turbidity, suspended solids and COD were determined .
-- Advanced particle removal techniques - Experiments on direct wastewater filtration with coarse sand filters and membranes, as well as tests with flotation of raw municipal wastewater were conducted. With these research activities, advanced techniques for particle removal could be developed and implemented in future, alternative wastewater treatment scenarios   .
-- Nitrogen removal by ion exchange - For a proper nitrogen removal in case of a low carbon/nitrogen ratio (BOD/N < 2.5), an inventory and evaluation study into the application of nitrogen removal by ion exchange is being carried out.
Natural (zeolites) and artificial (resins) ion-exchangers are being tested on wastewater. The opportunities for applying ion-exchange techniques in wastewater treatment are mainly determined by a fast, compact and inexpensive regeneration and brine treatment. Therefore, further research will also focus on the post-treatment of the concentrated regeneration liquid .
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Arjen F. van Nieuwenhuijzen is a scientific researcher at the Department of Sanitary Engineering at the Delft University of Technology. Contact address: PO Box 5048 - NL 2600 GA Delft - The Netherlands - phone: +31 15 278 4734 - e-mail: firstname.lastname@example.org
Adriaan R. Mels was employed at the Department of Environmental Engineering at Wageningen University and is currently working as a senior researcher for the Lettinga Associates Foundation
Patty Gerhardt-Meilink was with Kemira Kemwater Netherlands and is currently employed as project manager at Essent Water - South West Netherlands