Matamata WTP upgrade - Producing water fit for hobbits
Annual Conference
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In July 2007 Matamata Piako District Council (MPDC) received an E grading from the MoH for their 5ML/d Matamata WTP. This paper details the unique path that MPDC followed using a combination of innovative process design and in-house knowledge to turn an E grade plant into an A grade plant at the lowest possible cost to their rate-payers.
The catchment for the plant is in the Kaimai ranges. The water is usually of pristine quality (UVT>93%,<1.0NTU) however, the steep topography and heavy forestation results in extremely rapid deterioration in quality following rainfall. After heavy rain events the dissolved organic content of the water increases dramatically for 1-2 days (UVT<60%, 4NTU).
The existing plant was originally a manually operated and controlled direct filtration plant, followed by pH correction using lime and chlorination using chlorine gas. In order to comply with DWSNZ2005 (2008) and achieve an A grading the plant had to be modified to provide 3 Log of protozoal treatment.
The process selected for providing the 3 Log treatment was UV irradiation. However, in order for UV to be credited with 3 Log, certain pre-treatment conditions must be met. The most significant of these are for a turbidity of <1.0NTU for 95% of the time and a UVT of not less than 80%. The challenge therefore was to achieve sufficient dissolved organics removal following rainfall events to ensure that the UV pre-treatment criteria were always met. The upgrade design focused on the best way to achieve this objective utilising the existing assets.
Full-scale trials showed that direct filtration could achieve the required treatment aims during rainfall events provided the coagulant dose and flocculation pH were adjusted to respond to the changing source conditions. Once performance had been validated it was possible to optimise the design of the existing direct filtration plant.
The unique design innovation that allowed the process to work under all conditions whilst minimising the operational cost was the installation of an S::can on-line UV-Vis spectrophotometer configured with two custom algorithms. The first started the coagulant and poly dosing when the source water UVT dropped below 91% and stopped the dosing when it went back above 92%. The second, which operated once the coagulant dosing was started, automatically controlled the coagulant dose by using the predictive dose control package com::pass. Operating the plant in this manner meant that coagulant and poly are only dosed for approx 25% of the time, resulting in large chemical cost savings.
Other design innovations employed were converting an old leaf settling tank into a flocculation tank, replacing the old filter sand with single media filter coal beds thereby removing the need for replacing the filter floors and installing UV units on the outlets of each individual filter (3 No.). This had the advantage of providing inherent redundancy and enabled the plant to continue running throughout the UV installation.
The in-house knowledge was provided by Kaimai Valley Services (KVS) a business unit of MPDC who are responsible for operating the water treatment plants. KVS did all of the installation work and this gave the operators a real sense of ownership and pride in the upgraded plant.