ASSESSMENT OF THE FIRST YEAR OF OPERATION OF THE UF/NF WHITEMARK WTP
Evaluation of the performance of the new Whitemark WTP
R Tarr, P Kafieris
Publication Date (Web): 27 April 2018
While low pressure membrane processes are increasingly being used in water treatment for particulate and pathogen removal, removing dissolved contaminants such as NOM, iron and manganese is commonly addressed with physiochemical processes including coagulation and oxidation. For applications where chemical dosing and sludge production is undesirable however, nanofiltration can prove a viable alternative.
Nanofiltration (NF) membranes are capable of selectively removing problem compounds or ions, while retaining some hardness and most monovalent ions. Large molecular weight organic compounds are readily rejected while lower rejection of monovalent ions provides moderate permeate TDS levels and lower concentrate salinity for environmental discharge.
At Whitemark on Flinders Island, Tasmania, a new water treatment plant (WTP) combines ultrafiltration (UF) with spiral-wound NF membranes without the need for chemical pretreatment or oxidative processes. The UF provides particulate and pathogen rejection and feeds low SDI water to the NF system which achieves near-total colour and NOM removal. Furthermore the NF eliminates lead from the treated water, where levels previously exceeding ADWG health limits led to a public health alert to not consume.
The Whitemark WTP process consists of raw water straining, UF, NF, pH/alkalinity adjustment via calcite filters, GAC adsorption, UV and chlorination. A 70% NF recovery rate combined with no pre-treatment chemicals resulted in chemical-free waste returned into Pats Creek.
The WTP recently completed its first year of operation. Moderate raw water quality provided for stable initial performance, however, over the summer/autumn 2017 period, prolonged lack of rainfall, combined with major network leaks, resulted in a water shortage crisis. With insufficient storage to see out the annual dry period, concentrate discharge was redirected to Pats Weir for blending and retreatment. Water cartage also commenced from the Lady Barron WTP (the only other WTP facility on Flinders Island). The feed TDS increased from around 700S/cm to over 1400S/cm over a two to three-month period. Thanks to these measures, late autumn rainfall, and a leak repair program which decreased network consumption by over 50%, the crisis was alleviated prior to supply interruptions.
From March to May 2017 the impact of NF concentrate recycling became noticeable both in terms of EC and normalised DP. The NF recovery rate was decreased to 67% as the raw water quality and NF performance continued to deteriorate, until improvements began in May 2017. An intensive cleaning cycle further improved NF performance.
The NF membranes provide complete removal of true colour and DOC and the downstream GAC media only encounters trace levels of organics, significantly prolonging the expected media life and preventing the formation of THMs or DBPs. Dissolved metals are similarly eliminated without the need for oxidation or chemical dosing.
Following network cleaning and WTP commissioning, no pathogens have been detected in the network following commencement of supply. The Public Health Alert – Do Not Consume notice was lifted in November 2016, marking the first time water was deemed suitable for consumption in Whitemark.
Over the first year the Whitemark WTP averaged 198kL/d treated water at a WTP recovery of 63.8%. UF recovery was 92.4% while NF averaged 69.2% recovery. Specific energy consumption was 1.24kWhr/kL and chemical consumption was far below expectations.
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