Nitrate removal: Multi-zone ion exchange with less waste

An IChemE-award winning ion exchange system which removes nitrate from groundwater with minimum use for chemicals, reduced power consumption and waste volumes has been installed by ACWA Services at a number of UK water treatment works.

Figure 1. Batch versus continuous processing
Figure 1. Batch versus continuous processing
Figure 2. The Nitreat™ valve index cycle shows the treatment process.
Figure 2. The Nitreat™ valve index cycle shows the treatment process.
Figure 3. The installed NITREAT™ process which removes nitrates from drinking water.
Figure 3. The installed NITREAT™ process which removes nitrates from drinking water.

Historically, conventional ion exchange nitrate removal systems in the UK have utilised large resin-filled fixed beds operating on a duty/standby basis. Raw water is allowed to flow through the duty bed for a defined period of time – depending on the level of nitrate – before they are taken off-line for regeneration. In contrast, ACWA's NITREAT™ system uses a number of smaller vessels in which conventional nitrate-selective resin approved by the UK Drinking Water Inspectorate is employed in a counter-current ion exchange system.

During early on-site performance tests, the system produced waste volumes typically of less than 0.5% of total works output and the effluent was produced as a continuous low-flow stream rather than in periodic high-flow ‘slugs'.

NITREAT™ nitrate removal technology was developed by ACWA Services in association with PuriTech and was first introduced in the UK at the Sheafhouse WTW of Thames Water in 2005 – when all performance guarantees for power consumption, waste production and chemical usage were achieved. Since that time, eight systems have been installed for Anglian Water in the UK and are currently providing substantial cost-saving benefits. A further two systems for this company are well on the way to completion with one more in the design phase. 

The nitrate removal process

Adsorption and de-sorption involves mass transfer between a liquid and a solid phase. Traditional ion exchange is a dynamic process in which characteristic mass transfer profiles are established within the adsorbent bed. This is referred to as the zone of mass transfer (MTZ). For any set of operating conditions the MTZ has a characteristic length – the MTZL. The MTZL describes the kinetics of the process. In traditional systems the bed is run to exhaustion (nitrates beginning to break through) and then is taken off-line for regeneration.

The type of ion exchange offered by ACWA has adsorption and de-sorption occurring continuously and simultaneously within each process train. Several smaller beds are arranged in parallel, so that the total length of the series contact is more than the MTZL. Figure 1 shows the basic concept of batch versus continuous processing. Exhausted beds move out of the cascade at one end while newly regenerated beds come into the cascade at the other end.

The process cycle

The NITREAT™ process is divided into four separate zones, with each operating continuously. The NITREAT™ system can be broken up into four distinctive zones with all zones operating continuously and simultaneously. Typically, there are 20 columns installed, with 14 in the adsorption zone at any one time, one in displacement/backwash, three in regeneration and two in regeneration rinse.

The process is controlled by the patented multiport valve. The valve contains a unique process disc which has several process channels machined in it which separate and direct the various process flows within the valve.

The position or quantity of columns contained within each zone does not change physically. However as the multi-port valve ‘indexes' to the next process position, each vessel is effectively moved forward to the next position in the process cycle. Over a process cycle each vessel is taken through the entire adsorption zone, into displacement, through the regeneration and regeneration rinse zones, before returning to adsorption.

In the index cycle diagram (Figure 1), the valve position shows column number one at the end of the adsorption zone and completely saturated with nitrates, whilst column number 14 is at the beginning of the adsorption zone and barely loaded. 

The adsorption zone (process positions No 1–14)

This is the adsorption zone where the resin is loaded with nitrates and other anions (sulphate, bicarbonate) with all the vessels being operated in parallel.

The main feed port on top of the multiport valve is connected to the process disc feed channel which is at any time connected to 14 consecutive valve exit ports (14 × vessel feeds). The raw water passes into the valve and out to these 14 vessels, flowing down through the resin.

The nitrate-free treated water exits the bottom of the 14 vessels and passes back to the valve through 14 mating inlet ports. These are all connected to the treated water channel within the process disc which directs the flow out through a treated water port on the underside of the valve.

Depending on the works flow and nitrate loading of the raw water, the valve ‘indexes' the vessels periodically, bringing a new vessel into the zone and moving one out of the zone. An index consists of a precise 18-degree clockwise movement of the process disc within the valve body. No movement is evident externally.

In one example, the prevailing conditions determine that the valve will index once per 60 minutes and that an index is about to occur. It is then evident that one of the 14 vessels will have been in the zone for an hour, one for two hours, one for three hours, and one for 14 hours. The ‘freshest' vessel will be producing almost nitrate free water, whilst at the other end of the zone the vessel which has been in the zone longest will be exhausted of nitrate removal capacity and will be producing water with up to 10mg/l NO3. However, as an average over the whole zone the treated water is guaranteed to contain less than 5mg/l NO3.

Upon the next multi-port valve index, the ‘exhausted' vessel, saturated with nitrates is moved out of the adsorption zone and into the displacement zone. 

The displacement zone (process position No 20)

This zone only has one vessel in it at any time and it will contain resin saturated with nitrates. The vessel will have been treating raw water for a total of 14 indexes and may have accumulated some debris on the top of the resin during this period. As it passes into the zone the vessel will contain raw water, which may be relatively high in hardness salts.

This zone is the only one where the flow from the multiport valve is directed to the bottom of the vessel. Softened water enters through a port on the underside of the multiport valve and is directed to the port feeding the displacement zone.

The zone has a triple purpose:

• The upflow configuration fluidises the resin bed so that compaction and channelling within the resin bed is minimised ensuring a uniform bed for the next cycle of adsorption.

• The fact that the bed is fluidised ensures that any accumulated debris is backwashed out of the vessels to waste. This contributes to the maximisation of resin life.

• The hard water present in the vessel is displaced by soft water ensuring that in the next zone hardness salts are not precipitated on contacted with the brine regenerant. This minimises maintenance downtime for cleaning scaled equipment.

Upon the next multi-port valve index, the ‘backwashed' vessel, full of clean soft water, is moved out of the displacement zone sand into the regeneration zone. 

Regeneration & regeneration rinse zones (process position 15–19)

The remaining five vessels are operated in series but are split into two zones. The resin vessels move ‘right to left' through the five available positions, whilst the liquids flow ‘left to right'. This makes full use of the counter-current advantages improving the utilisation of the brine and allowing higher resin capacities. 

Liquid flow (From position 15 through 16, 17, 18, 19 and then to waste)

Soft water passes into the top of the multiport valve, through the ‘regeneration rinse' channel in the process disc and out to process position number 15, entering the top of the vessel and flowing down through the resin and out through the bottom of the vessel.

The flow passes back to the multiport valve and immediately back out to the top of the vessel in process position number 16 flowing down through the resin and out through the bottom of the vessel.

The flow passes back to the multiport valve at which point it is mixed with a 26% brine stream which enters the top of the multiport valve through another dedicated port. The brine is diluted by the soft water flow to 8% and is directed out of the valve to the top of the vessel in process position number 17, flowing down through the resin and out through the bottom of the vessel.

The flow passes back to the multiport valve and immediately back out to the top of the vessel in process position number 18 flowing down through the resin and out through the bottom of the vessel.

The flow passes back to the multiport valve and immediately back out to the top of the vessel in process position number 19 flowing down through the resin and out through the bottom of the vessel, back to the multiport valve and out to waste via a port on the underside of the valve. 

Resin flow – from position 19 through 18, 17, 16, 15 and back into position 14 at the start of the adsorption zone

The direction of ‘flow' of the resin is counter to that of the liquids through these two zones.

In the regeneration zone exhausted resin passes from the displacement zone (process position 20) through process positions 19, 18, 17 contacting with more and more concentrated brine solution as it does so. The nitrates, sulphates and other anions present on the resin are replaced with chloride ions.

The waste produced in this zone is predominantly sodium nitrates and sulphates and excess brine. A portion of this effluent (equivalent to the first 15% of the waste flow immediately following each valve index) may be recycled back to the bulk brine tank thereby reducing the amount of brine consumed and the amount of waste produced.

Upon the next multi-port valve index, the ‘fully regenerated' vessel in process position 17, full of clean brine, is moved out of the regeneration zone sand into the regeneration rinse zone.

This zone consists of the remaining two process positions and is employed to prevent brine from entering the treated water stream. The bulk of the brine is flushed out whist in process position 16 and whilst in process position 15 the resin is polished to the highest possible quality before moving back into the adsorption zone. 

Waste recovery

Waste from the NITREAT™ process is relatively low due to the recovery and reuse of process effluent and is a significant factor in the reduction of disposal costs. As a large proportion of the displacement flow may be recovered, waste effluent mainly comprises the rinse/brine flow, with instrument, self-cleaning filter waste and water softener waste adding a very small fraction to the overall volume. Up to 15% of waste brine may be recovered – the actual fraction being site specific.

The NITREAT™ process design criteria has proved to be extremely effective with regards to the efficient removal of nitrates, low waste production, low salt usage and low power consumption requiring low operator intervention on mainly unmanned sites.

Regeneration has been efficient and reliable – with sampling data confirming that brine has been fully flushed out of the freshly regenerated vessels prior to re-entering service as evidenced by an extremely consistent quality of treated water.

Recently, Yorkshire Water in the UK adopted the NITREAT™ technology as the best solution for use at its Keldgate WTW. The new plant, which will be installed by ACWA, is scheduled for commissioning by the end of 2009 and will be the largest of its kind in the UK water industry.

The works capacity at Keldgate is 90 million litres per day and the design will allow up to 33 million litres of it to pass through the nitrate removal system. After reducing nitrate levels to fewer than five parts per million, the treated water will be blended back into the main flow to produce water for public consumption at 42mg/litre – well within the limits for drinking water quality and future proofing the system against anticipated rises in nitrate levels.