Upgrading waste water systems discharging to rivers and lakes will be expensive and those costs may fall disproportionately on smaller communities, a study for the government’s three waters review says.

While most of the country is served by sewage and waste water systems discharging to the sea, about 5 percent of the population relies on waste systems discharging to land and another 13 percent on schemes discharging to fresh water.

Consulting engineers GHD and Boffa Miskell assessed 145 publicly-owned schemes that will require upgrading to meet the new national policy statement on fresh water quality. Upgrading them so their receiving waters could meet a B classification for E.coli, nitrates and ammonia levels would cost between $1.4 billion and $2.1 billion.

But more than a third of the schemes serve communities with fewer than 500 people, while more than 80 percent serve communities of fewer than 5,000. The annual cost of the upgrades for users of the smallest schemes was close to $3,600 a year – five-times that of the largest.

Looked at regionally, schemes in Taranaki and on the West Coast had the highest average annual costs at $2,929 and $2,315 per household respectively.

Local government minister Nanaia Mahuta said the latest report adds to the picture of looming costs that will come with improving drinking water, stormwater and waste water management.

“This does not include the costs of upgrading infrastructure for discharging to beaches and coastal environments, nor the unknown but potentially even higher costs of preventing waste water pollution on beaches and in urban environments through stormwater overflows,” she said in a statement.

The study found that the only reliable way to meet the fresh water standard by 2025 was to upgrade the schemes to a system of biological nutrient removal with activated sludge plant and ultraviolet disinfection.

But it observed that going from relatively low-tech stabilisation ponds built 60 years ago to the new mechanised systems would require more skilled staff and that could pose a “significant barrier” to progress. Handling and disposing of sludge from such plants daily can also be difficult and expensive, the report noted.

The authors observed that water quality in the catchments the plants discharge into is also influenced by other discharges and land-use.

It found that the schemes having the biggest impact on water quality could probably be upgraded at a cost of $160 million to $240 million. Including schemes having a moderate impact would add another $630 million to $950 million to the bill.

The study didn’t estimate the cost of eliminating wet weather overflows, which tend to be highly variable and tend to be only a small contributor to contaminant load during such events.

It noted improved wet weather overflow would be more driven by the policy standards for public health and recognition of iwi values in fresh water management.

Were those factors costs to be included in the scheme upgrades that work would require more specific analysis of overflows.

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