Working with wastewater
The absence of overpowering odour is the first thing you notice at New Plymouth’s wastewater treatment plant.
It’s the sort of facility that people don’t really think about, but if it wasn’t here, New Plymouth could be swimming in millions of cubic metres of sewage a year.
Tomorrow the plant (WWTP), on Rifle Range Rd, will host the public to show what goes on behind closed gates.
There’s several steps in the process from flushing your toilet to the treated water returning to the ocean, NPDC Three Waters manager Mark Hall explained during a preview tour on Wednesday.
In a nutshell, the sewage is pumped in and screened and then goes through a grit separator – which removes truckloads of black sand.
Then it goes into bioreactors to undergo activated sludge aeration – essentially, be eaten by bugs while air is added.
There are two main basins filled with brownish water, and the bacteria appears like a scummy foam on the surface. In some areas the water looks like it’s boiling, bubbling away like a foul cauldron.
There is a some odour, depending on which part of the process you standing near, but thankfully most of the stench is piped away.
The water then goes through a clarifier, where the sludge sinks to the bottom, and is chlorinated on the way out.
Finally it goes chlorine contact through the tank, before making its way out to sea.
Solid waste from the treatment process is dried out and turned into fertiliser.
The figures are staggering: each year, the WWTP treats up to eight million cubic metres of wastewater – equivalent to four billion two-litre milk bottles – which comes from a number of sources including about 26,000 properties. At peak times, intake can go up to about 400-600 litres a second, while during big storms it has had flows up to 1165 litres a second.
New Plymouth has some of the cleanest treated wastewater in New Zealand – well within the standards for safe swimming and seafood gathering.
The site is computercontrolled, and people are on standby 24/7.
The power bill reaches about $535,000 a year for 4.4 million kilowatts of electricity – but before 2006 upgrades the plant used 5.1m kilowatts.
Things have changed since the site was built in 1984 – waste screening is now automatic, so workers don’t see the false teeth, the money, the driver’s licences and ‘‘lots of McDonald’s toys’’ that end up in the sewage system, Hall said.
However, golf balls are found in the basins, but they cannot have come through the sewage screens – the odd drive from the neighbouring Nga¯motu Golf Course clearly goes a long way.
Rubbish extracted from the water is compressed before going to landfill. In the screening area bits of wipes are visible in pipes through which the extracted waste is sent.
Wipes remain a huge problem, Hall says, because they don’t break up easily.
‘‘It’s still held together,’’ he explains, pointing to the fragments of wipes that are threaded through extracted rubbish and dangling from the pipe ends.
‘‘Toilet paper doesn’t do that,’’ he says – it will disintegrate almost straight away.
If there is one thing Hall wants to get across to people it’s that they have to take care of what they put into their sewers.
‘‘People can’t treat it rubbish bin,’’ he says.
‘‘There’s a person who ends up cleaning up if something goes wrong.
‘‘It’s a real person and they’re on the end of a shovel.’’ like a
The open day runs from 10am1pm tomorrow. Closed shoes are required but under-5s cannot attend for safety reasons.