Local Government Magazine
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Smart solutions – Wastewater management

Confronted by ageing or inadequate infrastructure, smart solutions to wastewater management are a priority for many councils. Local Government Magazine looks at four projects aimed at providing a more cost-effective and lasting outcome for the local bodies involved. By Patricia Moore.

Organisation WSP Opus

Project Mangawhai Wastewater Treatment Plant Upgrade

Local authority Kaipara District Council (KDC)

Project status At project definition stage

Mangawhai is a growing community with increasing demands on infrastructure, particularly during peak holiday periods. As technical principal wastewater engineer Andrew Springer explains, WSP Opus has been working with KDC to determine a critical timeline for the delivery of growth upgrades, considering aeration capacity, flow balancing, irrigation storage and distribution.

The approach has been to use a root cause process to establish the cause of issues at the site, then, through assessing the business risk for each issue, establish a priority ranking for investment. “The most critical aspects will be addressed first and, only if funding permits, lower priority aspects will be included.”

For KDC, this approach enables both informed decision-making on every known issue, whether growth or plant maintenance requirements, and tighter control on budget by addressing what’s really needed, with reduced likelihood of scope change, says Andrew.

“Solutions are focused on resolution of the cause of an issue and not on the symptoms. The risk management approach can be used to compare options and feed into site asset management plans.”

Andrew says this root cause process has been used by UK water companies for several years resulting in significant understanding of problems, directing the investment and avoiding unnecessary expenditure. “We are sharing this experience with New Zealand as a tool to help decision-making.”

Organisation Tonkin + Taylor

Project Auckland Safe Swim & Safe Networks Programme

Local authority Jointly Auckland Council Healthy Waters and Watercare

Project status Ongoing since November 2017

Tonkin + Taylor sector director water, Clint Cantrell, says this programme represents international best practice for the assessment and protection of bathing recreation waters in an urban setting.

“It’s a cutting-edge programme in terms of delivering real-time and forecasted water-quality information that allows the public to make informed decisions when choosing a location to swim.”

By allowing the community to make planned decisions on where to swim, the programme has already delivered substantial benefits, says Clint. “It’s also enabling Healthy Waters and Watercare to identify and mitigate specific causes of pollution that impact on swimming, such as failing septic systems, faulty private plumbing, connections and leakage through asset defects.”

Website analytics show a very high rate of community usage and, says Clint, recent surveys indicate that 73 percent of Aucklanders believe it is important to ‘check before you swim’.

In contrast to the traditional approach of compliance with prescriptive standards, the programme highlights the importance of focusing investment programmes on agreed community outcomes – with adequate science and evidence to link outcome effects to sources to drive a high return on capital investment.

“In Auckland it’s clear the community places a huge value on the ability to recreate safely in the region’s designated swimming locations.”

Organisation GHD

Project Greenfield Development Infrastructure

Local authority Various including Auckland, Hamilton and Tauranga

Project status Ongoing

The wastewater servicing of greenfield development projects is challenging engineers charged with designing systems that will service the entire future development while also catering for the initial stages. So says Robert White, northern water and wastewater manager with GHD.

“Developments can ultimately include thousands of properties but may have only a few connected on day one.”

He notes that major new developments are frequently on the outskirts of existing urban areas and require lengthy connecting infrastructure.

He says achieving self-cleansing velocities in gravity sewers, and minimising retention times in sewer rising mains and the network in general – without limiting potential for future growth – makes the design a four-dimensional problem.

“Councils need to be more aware of non-conventional solutions to minimise problems, including vacuum sewer systems, reduced diameter rising mains with high velocity / high-head pumps, twin rising mains, chemical dosing and forced ventilation through odour beds.”

Robert says twin asymmetrical rising mains often provide advantages over twin rising mains on the same site, allowing the smaller rising main to be used before switching to the larger main and ultimately using both. However, the pipes need to be correctly sized to work efficiently.

“Councils need to fully understand the added cost and nuisance associated with operating these systems.”

Engineers have recognised the need to find solutions to support the delivery of housing whilst providing the utility operator with a system that doesn’t create unwanted problems, says Robert.

“Designing and delivery wastewater systems that work well now and in the future is a real challenge.”

Organisation Stantec

Project Rosedale Thermal Hydrolysis Project, Auckland

Local authority Auckland CCO Watercare

Project status Ongoing

John Messenger, Stantec Australia’s global practice leader treatment Asia Pacific, says this will be the first full-scale thermal hydrolysis installation in New Zealand. His company is responsible for the preliminary and detailed design.

He says the thermal hydrolysis process is applied to wastewater sludge upstream of anaerobic digesters and has three significant impacts.

“Firstly, it increases the amount of sludge which is destroyed in the digestion process and thus produces more methane, which is available to generate electric power.

“In addition, it alters the properties of the sludge so that the digesters can treat much thicker sludge,” he says.

“This results in the local authority needing to build fewer new digesters in the future as the local population increases. It also causes the digested sludge to dewater to a much dryer cake which means that trucking costs decrease by about 40 percent.

“Finally,” he notes, “the high temperatures used to hydrolyse the sludge also serves to pasteurise it and this opens up more opportunities for beneficial re-use of the sludge on agricultural land.”

He adds that the proprietary thermal hydrolysis equipment, as illustrated by a Cambi B2 package unit at a plant in Ourense, Spain (see photo), is only part of the capital infrastructure required to implement the full technology.

There also needs to be a good understanding of the nature and footprint of the balance of plant. This might involve sludge thickening, pre-dewatering, co-generation and anaerobic digester modifications.

Working with a skilled and capable client organisation has allowed the design development to progress at pace but also benefit from operational knowledge. The ability of a client organisation to add value to a project cannot be underestimated. 

• Patricia Moore is a freelance writer. mch@xtra.co.nz

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