WATERMARK was commissioned to evaluate the potential  of a new 500 meter deep bore with an artesian flow of about 210 m3/hr as  a supplementary soft water source for the Wanganui water supply which is renowned for its hardness.  Another consideration was the bore’s  potential as a risk mitigating source of water because it was located distant from the main Kai-Iwi bores which are the primary source  of water for the City.

The water in the bore located in the suburb of Aramoho bore water was characterised by high colour (distinctly brown) low hardness (less than 50mg/l) high temperature  (about 19 degrees C) and an ammonia content of 2.5mg/l.  The preliminary study carried out by WATERMARK looked at implications of the  Drinking-water for NZ Standards, the condition of the network, the need to continue network chlorination and the very vocal message from the Community that there was a low tolerance for  ‘chlorine odour and    tastes‘ in the City water supply.

 

After a decision was made to treat the water to remove colour and ammonia WATERMARK carried out bench tests using five  potential colour and ammonia removal technologies including the traditional aeration technique. Bench scale aeration based on theoretical calculations showed promise but when pilot tested, ammonia stripping performance was only marginally successful. The site had significant restrictions on noise levels and this together with the fact that upgrading the electricity supply to the site was going to be expensive further investigations involving aeration were not considered.  Life-cycle costing based on the preliminary pilot trials also indicated that aeration was going to be one of the the most expensive options.

Of the other technologies, ozone showed promise.  Ozone is not normally recognized as an efficient oxidising agent for ammonia removal because of the long contact time required – in this case in  excess of 5-5 .5 hours  but further pilot trials  using an innovative combination  of pre- and post chlorination together with ozone addition resulted in a treatment concept that required less than 3 hours contact time –  significant saving on capital costs.

Performance specifications prepared by WATERMARK based on  the results of the pilot studies: pre and post chlorination and 3 hrs ozone contact  time were used by the client for the design the treatment plant.  The completed treatment system (200 m3/hr ) continues to provide the Aramoho suburb of Wanganui with soft ammonia and colour free water that is successfully blended with the hard water in the outer reaches of the Aramoho zone without producing complaints from customers about ‘chlorine tastes and odours’