Sewage treatment proves ‘fascinating’
Sewage treatment plant team leader Allan Jones says he sort of fell into the job.
‘‘Not literally,’’ he added with a laugh from his office on Bells Island, near Nelson.
It doesn’t smell bad in the office, nor around the five ponds.
However, there is an odour at the spot where the wastewater is pumped into the plant and undergoes its initial treatment – to separate the solids from the liquids.
Jones confirmed the separation was not done by hand ‘‘although the looks some people give you, it’s as if they think that’s what you do.
‘‘On a bad day, it can smell,’’ he said of his workplace. ‘‘You get that Rotorua-y type smell, that sulphur-y type smell but it doesn’t smell like s***.’’
Some people, though, couldn’t seem to handle being at the plant.
‘‘We’ve had people turn up here, they’ll get out of the truck and they’ll be on their hands and knees dry retching and they’ll get in, turn around and go away,’’ Jones said.
‘‘It’s real psychological.’’
Jones started working at the plant about 10 years ago as a mechanical engineer, maintaining the equipment that keeps it operational. Since then, he has undertaken additional training and now holds a diploma in wastewater treatment.
The Bells Island Wastewater Treatment Plant is operated by the Nelson Regional Sewerage Business Unit, which is jointly owned by the Nelson city and Tasman district councils.
It treats the grey water and toilet waste from homes at Tahunanui, Stoke, Richmond, Brightwater, Wakefield and Mapua. It also handles industrial wastewater from Alliance Nelson, ENZA Foods and Nelson Pine Industries.
Jones said the average flow into the plant was about 14,000 cubic metres a day. However, it could and had taken about four times that quantity in 100-year flood-type situations. ‘‘Usually, in a good rain, it might double,’’ he said of the flow rate.
After it arrived, the raw sewage was run through a drum screen to sift out the inorganic matter. ‘‘All the rags and knick- ers, false teeth, kids toys and adult toys,’’ Jones said. ‘‘It all gets screened out and goes to landfill.’’
Another machine removed grit such as fine, black sandy material that ran off the streets and could damage the equipment at the plant.
The sewage then went into the primary clarifier, a big chamber about six metres deep.
‘‘It lets all the suspended solids settle in the bottom where a big scraper scrapes it down into a big hopper and that gets taken away as a sludge, and what you have left is the clarified liquid,’’ Jones said. ‘‘It’s still got that dirty look to it [the liquid] but there’s no lumps in it.’’
The liquid from that primary clarifier then usually headed to the ‘‘activated sludge basin’’ where aerators created fine air bubbles that fed the bacteria present in the wastewater.
‘‘It promotes all the organisms to grow and they feed on all the soluble organic stuff that’s in the wastewater and they grow and multiply,’’ Jones said. ‘‘We’re essentially growing these big clumps of bugs.’’
Those ‘‘bugs’’ dropped out and settled down as a sludge during the next stage, a secondary clarifier, and the liquid that remained ‘‘you could almost drink it – it comes off nearly crystal clear’’.
From there, that liquid headed to one of three 10ha facultative ponds where algae ‘‘breaks down the remaining load’’.
The liquid also underwent UV treatment from the sun, particularly in the two maturation ponds that followed. Once that process was complete, the now fully treated liquid was released into the Waimea estuary after high tide.
The sludge, meanwhile, from both clarifiers was thickened ‘‘so it’s like lumpy porridge’’. It was then fed into one of six large auto thermophilic aerobic digestion (ATAD) tanks. Aerators pumped air into the sludge.
‘‘It has the exact same effect as it does in the aeration basin,’’ Jones said.
‘‘By putting air in, it promotes the growth of a certain type of bacteria – the thermophilic bacteria.’’
That bacteria fed on the organic matter in the sludge and the tanks reached about 70 degrees Celsius.
The sludge was in the ATAD tanks for about 14 days during which time ‘‘it’s essentially been pasteurised so all the pathogens and all that nasty stuff you don’t want has been wiped out’’. The thermophilic bacteria itself thrived in temperatures above 40C so it was no threat once the treated sludge was returned to the ambient outdoor temperature.
The fully treated sludge was then pumped as biosolids through a 3.5km pipeline to Rabbit Island where it was used as fertiliser.
‘‘What comes out is class-A biosolids,’’ Jones said. ‘‘It’s as good as you can get and that means it can go straight onto land.’’
The whole treatment process on Bells Island averaged about 60 days. Samples were taken at every step.
Jones said he worked with an awesome team in a role that was ‘‘absolutely fascinating’’.
‘‘It’s a mix of everything. There’s biology and chemistry, and there’s mechanical engineering,’’ he said. ‘‘Essentially, we’re taking a natural process and we’re just accelerating it. We’re containing it, condensing it and accelerating it.’’