Wedeco ozone and UV plant at Bryn Colwyd

Bryn Colwyd WTW supplies water to 76,000 people in Colwyn Bay and Llandudno on the north Wales coast.

The plant receives its supplies from the Lyn Colwyd reservoir in Snowdonia and the raw water supplying the works has a low level of turbidity, some colour and moderate levels of organic carbon.

The distribution system is extensive with residence times in excess of 48hrs and much of it being old, problems existed with the maintenance of effective residual disinfectants. The potential for biological re-growth in distribution was also significant.

Following pilot scale trials, Hyder Consulting selected a process involving ozonation followed by GAC pressure filtration as the optimum solution to fulfil water quality and budgetary criteria.

A major advantage of ozone-GAC treatment can be summarised as follows:

• Ease of operation and control compared with coagulation and filtration methods
• Elimination of waste sludge production
• Removal of assimilable organic carbon (AOC)

In order to minimise the requirement for storage and dosing of chemicals, low pressure UV tubes were selected for primary disinfection. This happens downstream of the GAC absorbers.

The raw water enters the works at up to 19 bar pressure and it was a requirement to design the treatment process such that the works outlet pressure was sufficient to supply the service reservoirs without additional pumping.

Working in close co-operation with Hyder Consulting, Wedeco developed a novel ozone introduction and contacting system which was both compact and capable of operating at pressures up to 25 bar. It was also necessary to design the UV reactor vessels to operate at these pressures.

Since the plant was put into service, Hyder’s customers have seen a substantial improvement in water quality resulting in a reduction in the number of complaints.

UV plant

Figure 1 shows the arrangement of the UV disinfection system within the plant. Three stainless steel pressure vessel chambers are arranged in parallel to disinfect the water leaving the GAC absorbers.

Each chamber is fitted with three banks of eight UV lamps, arranged perpendicularly to the water flow, where each lamp is housed in its own quartz sleeve.

This arrangement allows the operator to replace the UV lamps even if the chambers remain pressurised. Water flow through the chambers is controlled by a motorized outlet valve.

Due to the high operating pressure and flows, the mechanical design of the chamber vessel walls was adapted to ensure that flexing of the vessel would be kept to a minimum during all probable conditions.

The possibility of pressure surges or water hammer also had to be taken into account, both in the mechanical design of the vessel where diffuser plates are fitted at both the inlet and outlet of the vessel and in the control of the chambers’ motorized outlet valve.

Each chamber is able to treat a water flow of up to 250l/s, thus for normal flow conditions two of the three reactors are in operation with the third available as a standby unit.

The UV dose applied to the water is inversely proportional to the water flow rate through the reactor; a high water flow will result in a low dose and vice versa.

Each chamber is controlled and monitored by a remote PLC system which communicates all relevant information to the plant’s own SCADA and telemetry systems.

The UV system has been designed such that in any adverse conditions, the system can still be operated manually.

The UV PLC receives signals representing the plant water flow rate and UV intensity signals from each bank of lamps within the chambers.

Using these signals it can calculate the UV dose that the water passing though the chamber is receiving.

When the PLC asks a chamber to start, the following sequence will occur;

1/ the UV lamp filaments are pre-heated to get them to operational temperature and then the arc is struck across the lamp. The PLC is able to detect that a lamp has failed to strike and will initiate a restart on that failed bank of lamps.

This could in theory occur when very cold water is being admitted from the upland reservoir in winter.

2/ When the lamps are operating the system opens the outlet motorized valves and allows water flow through the UV dosing system.

3/ If the water flow rate increases such that the dosing rate falls towards the minimum allowed, then the PLC will start a second reactor to split the water flow between reactors thus providing additional UV dosing capability.

Ozone plant

The ozone dosing plant at Bryn Colwyd is probably unique in the UK. The ozone plant was designed to dose a controlled amount of ozone into an incoming water supply at a very high pressure of 20 bar.

The ozone dosing is to remove colour from the incoming water prior to the GAC absorbers, thereby reducing the filtration loading.

The colour of the incoming water from the upland reservoir is very variable although a distinct peak occurs in the autumn when it can exceed 24 Hazen.

The colour set point for the ozone system is to dose ozone such that the water colour entering the GAC absorbers is just 6 Hazen.

Figure 2 shows the ozone dosing arrangement. The dosing system is set up to operate as duty / assist / standby.

Ozone gas is manufactured from an external LOX supply, by Wedeco ozone generators.

The ozone system derives its motive water and cooling water for the ozone generators from the filtered and disinfected supply leaving the UV chambers.

The motive water (at 17 bar) enters an ejector where the ozone/oxygen gas mixture is introduced into the motive water. The gas flow rate into the ejector is controlled by the ozone PLC to provide the ozone dose needed to reduce the water colour prior to the GAC filters to the colour set point.

The passage of water through the ejector reduces the water pressure at its outlet to approx. 7 Bar. This heavily ozonated water then enters a de-gassing vessel where the off-gas (that which has not been absorbed by the water) accumulates under pressure in the head space of the vessel.

The de-gassing vessel is vital to prevent any de-gassing occurring within other parts of the works, or when the pressure declines in the distribution system outside the works.

The ozonated water in the de-gassing vessel is then pressurized back up to 25 bar by a pressure pump. The pump injects the heavily ozonated water back through a water flow control valve (which is controlled by the water level within the de-gassing vessel), a non return valve and finally back into the works inlet water, where it mixes and reacts to remove colour from the inlet water.

The off-gas within the head space of the de-gassing vessel has to be vented to maintain a system pressure equilibrium.

However, the ozone content of the off-gas must be destroyed before the venting.

A pressure sustaining valve on the inlet to the catalytic ozone destructor maintains gas pressure within the head space of the vessel, while the sustaining valve allows the over pressure off-gas to pass through into the destructor and then to the atmosphere.

The ozone system is controlled and monitored by an ozone PLC, configured from two individual PLCs as a duty/ standby PLC arrangement.

The ozone PLC provides all of the relevant information on the status of the ozone plant to the WTW’s SCADA and telemetry systems.