Innovation harnessed in Dutch water treatment
A utility in the Netherlands has taken an innovative approach to making treatment more effective and more sustainable at a number of its sites. Aqualia's Frank Rogalla finds out what has been achieved
PWN is a social enterprise operating on a non-profit basis. Each year, 110Mm3 of drinking water is supplied to 1.5M people with about 750,000 connections for private households, companies and institutions at cost price.
Water treatment takes place in four water treatment plants (WTPs) with innovative features:
- Huizen & Laren: draws water straight from an underground well - all harmful compounds and bacteria have been filtered out during the passage through soil
- Bergen & Mensink: takes surface water from the River Rhine and the Ijssel Lake, which is first strained at the source. Particles of heavy metals and other pollutants are removed by means of flocculation and settling. This treated water is infiltrated into the dunes to filter harmful compounds and bacteria. After several weeks, the water is pumped back up and any remaining particles are filtered again
- Andijk: takes surface water from the Ijssel Lake which passes through micro strains, and flocculation, followed by sand filtration. Then it is treated with UV/H2O2, with the first integrated purifying installation in the world combining UV light and hydrogen peroxide for surface water, put into operation in 2004. Carbon filters then remove any harmful compounds
- Heemskerk: In addition to the new UV/H2O2 purification in combination with activated carbon started up in 2008, the plant at Heemskerk combines ultrafiltration (UF) to screen bacteria and viruses with reverse osmosis (RO) to block salts and organic compounds, such as pesticides. The resulting product water is absolutely pure, but needs to be blended for corrosion protection and taste improvement with infiltration water pumped up from the sand dunes
Heemskerk WTP was commissioned in late 1999 and features the first integrated membrane purification for surface water in the world, applying UF and RO to a capacity of 55,000m3/d. During the pilot phase it became clear that Lake Ijssel water had a very high fouling potential for UF.
Extended pre-treatment (coagulation, sedimentation, rapid sand filtration and activated carbon filtration) was needed to achieve a stable operation at a relatively high flux rate of 113l/h/m2. The downstream RO plant was running with a flux of 28l/m2/h, without any chemical cleaning in the first three years, only dosing small amounts of anti-scalants to prevent fouling.
Research into the fouling mechanisms at Heemskerk continued, to improve the pre-treatment for Lake Ijssel water, not only for the Heemskerk plant, but also for additional direct treatment in Andijk. Experiments carried out in parallel with in-line coagulation and treating the raw water directly with UF membranes showed that irreversible fouling is not caused by pore blockage, but by attachment of the deposited material.
Colloidal polymers seemed to be formed by natural organic matter (NOM) of Lake Ijssel and organic metal complexes. The interaction of these polymer films with the membrane material, which is hydrophilic and negatively charged, made it difficult to reverse fouling.
This NOM-fouling of UF-membranes is very often attributed to the deposition of high molecular weight organics (HMW), like polysaccharides, on the membrane surface, organics larger than the molecular weight cut-off value of the UF membrane.
In some cases, rejection of smaller organics like carboxylic acids and humics was measured and attributed to the deposition of polysaccharides on the membrane surface, making the "pores" of the membrane smaller to reach the cut off value of the acids and humics.
As high concentrations of the larger organics interact at the membrane surface and form long polymers, the low molecular weight (LMW) organics, like carboxylic acids, and humics combine with the HMW organics by electrostatic forces and accelerate the formation of a film the same way organic metal complexes do. The membrane may rapidly become irreversibly fouled if the formed film and the membrane are oppositely charged, favouring the adsorption of the film on the membrane.
Two solutions exist to reduce the fouling potential of Lake Ijssel. The first is to remove LMW organics, like the hydrophilic acids and humics, to avoid organic metal complexes, by using ion exchange to prevent fast polymerisation and adsorption to the membrane surface. The second is to reduce the surface charge of the membrane or increase opposite charge to promote electrostatic exclusion of the formed film.
UF was considered to be a promising solution for treatment of Lake Ijssel water. The presence and nature of NOM in the raw water was studied to find a relationship between the properties of the membrane and the NOM, so that the fouling problem can be better understood and the most suitable membrane material can be applied. The main purpose is to improve the efficiency of the advanced oxidation process (AOP) and minimise the production of assimilable organic carbon (AOC) by achieving the following objectives:
- Increase the UV-transmission (UVT)
- Improve the removal of dissolved organic carbon (DOC)
- Remove nitrate to lower the formation of nitrite;
- Total removal of suspended and colloidal matter independent of the feed water quality.
To remove the LMW fractions, an anion resin was tested, which resulted in a high gross flux rate on the UF with almost no fouling, and also increased UVT considerably and removed a large amount of nitrate and DOC. Manufactured and commercialised by Orica in Australia, a very fine resin, magnetic ion exchange (MIEX) was dosed into the feed water and settled after sufficient time in contact reactors.
The magnetic properties of the resin particles enhance the settling which makes it possible to use very small resin particles with a relatively high specific surface area, resulting in better kinetics and lower resin inventories. This process is especially developed to treat waters containing high levels of suspended and colloidal matter and is until now the only ion exchange process that has been used on a large-scale to remove DOC as a first treatment step.
To make the process more attractive for PWN, an alternative ion exchange process to treat surface water directly was developed, allowing the use of other commercially available non magnetic resins, and including a single pass adsorption process. The major part of the magnetic settled resin (90-95%) is re-circulated without regeneration, resulting in large detention times of the resin and possible changing adsorption kinetics and 'blinding' of the resin by a biofilm.
In contrast, the new approach should allow a complete regeneration of 100% of the resin before reuse to overcome the formation of a biofilm.
Test results with suspended resin indicated almost immediately that the use of 'fresh' resin, after re-generation led to the highest possible removal of DOC (analogous to increase in UVT) at the lowest resin concentration, leading to lower resin inventory, higher removal rates and lower contact times compared to recirculated resin. This improvement of removal kinetics led to a 50% smaller footprint of the system compared to the MIEX process, thanks to a smaller contactor and settler volume, and allowed the use of a wide range of non-magnetic resins.
A continuous flow model based on batch test data from various resins allows the control of the resin concentration in the reactor with sensors, measuring UVT and DOC in the feed water and the product, to make the process very tolerant to flow variations induced by water demand or changing water quality.
In total six different anion resins have been modelled from three different suppliers (Orica, Rohm & Haas and Lanxess). From these six resins, three have been tested within the pilot leading to a final choice for a long-term trial to fine tune all operating parameters, including the regeneration and the reuse of the regeneration fluid.
Since the resin is not completely loaded before regeneration, desorption can take place at lower salt concentrations (30-60g/l instead of 120g/l or more), with lower volumes and shorter contact times leading overall to a lower salt usage.
As a result of these investigations, an innovative pre-treatment facility is to be built in Andijk to replace the existing treatment plant. The 3,200m3/h plant will be expanded to 5,000m3/h. Andijk 3, as it will be called, will combine a new pre-treatment process based on suspended ion exchange (SIX) with new CeraMac ceramic membranes to form a filter that blocks all organic substances.
This process is expected to result in higher water quality and lower energy consumption with a lower environmental burden. Before the introduction of its SIX and CeraMac technologies, up to 5,000t/y of iron chloride sulphate and up to 6,000t/y of sodium hydroxide were added as an agent to the pre-treatment process for flocculation prior to filtration.
The new technique eliminates the use of iron, using a reusable ion exchange resin instead. Compared to existing, conventional water treatment facilities, it appears that the new system will deliver higher water quality in a more sustainable way. Moreover, owing to the higher quality of pre-treated water, less energy is required for the remaining treatment steps in the drinking water production process.
Although ceramic membranes are widely used in industries such as in food processing, chemical and pulp and paper, their application in drinking water treatment and water reuse is fairly new and has been limited mainly to Japan. Metawater has installed about 100 plants treating a total of 500,000m3/d. The largest installation, with a design capacity of 171,070m3/d is currently under construction in Yokohama.
Ceramic membranes offer several advantages over polymeric membranes, including:
- No fibre breakage and long membrane life (>15 years)
- Very strong resistant to a wide range of cleaning chemicals
- Withstand higher pressures up to 5 bar
- Water recovery of more than 98% because of dead-end filtration and low backwash frequency
- Power consumption of less than 0.1kWh/m3
CeraMac is an innovative block design that replaces membrane modules in individual stainless steel casings by a single stainless steel vessel with up to 200 ceramic elements. This reduces the capital cost and operating cost by about 20% due to reduced quantities of stainless steel, valves and other instrumentation, and also, the smaller footprint required.
The resulting savings make the set-up and operating costs of ceramic membranes cost-competitive with polymeric membranes.
While the treatment facility in Andijk will be completed in 2013, the water treatment technology will also be tested in close partnership with the Public Utilities Board of Singapore. A first ceramic membrane demonstration plant for 1200m3/d will be built at the Choa Chu Kang Waterworks for £2.54M to check its performance and system optimisation during a 18 month trial.
Although the initial capital cost of ceramic membranes is comparable to polymeric membranes, in the long run it might be cheaper as their lifespan is estimated to be about 20 years - four times longer than polymeric membranes that have been changed every five years in the reuse facilities in Singapore.
The first pilot trial of the ceramic membrane technology in Singapore was conducted at Bedok NEWater factory from 2007-09. The study indicated that coupled with a pre-treatment coagulation process, the ceramic membrane had a better performance in terms of trans-membrane pressure (TMP).
A ceramic membrane performs best if ozone is added to the system because it stabilises the TMP and destroys micro-pollutants.