DEMM Engineering & Manufacturing - - FRONT PAGE - By Steve Hart

Sel­wyn coun­cil had a prob­lem in 2011. Fast-grow­ing com­mu­ni­ties in Preb­ble­ton, Lin­coln and Rolle­ston were putting pres­sure on its waste man­age­ment sys­tem, and there was an in­flux of peo­ple as a re­sult of the Christchurch earth­quakes.

De­cid­ing there had to be a bet­ter so­lu­tion than mov­ing tonnes of sludge by road from its ef­flu­ent treat­ment plant in Eastern Sel­wyn to a land­fill, the coun­cil asked MWH New Zealand to look at the is­sue.

The firm re­designed the orig­i­nal plant to ex­pand it as a waste sludge grav­ity thick­ener and aer­o­bic di­gester, at­tached a de­wa­ter­ing plant, and so­lar air dry­ing hall that in­cludes a ro­botic sludge turner.

Rainer Hoff­mann, the firm’s chief process engi­neer, says: “The so­lar drier is the first in New Zealand. It is a test to see whether it can work with the South Is­land’s cooler cli­matic con­di­tions in win­ter. These sys­tems are used in Aus­tralia, but it is hot­ter there. But the Ger­man­made sys­tem is used across Europe. Re­sults so far prove the idea works well.

“The so­lar drier works on nat­u­ral heat and ven­ti­la­tion. The biomass is still quite ac­tive, so there is heat gen­er­a­tion, and as it is turned by the sludge man­ager it brings in oxy­gen, and in that process you gen­er­ate more heat, which kills off lots of the pathogens.”

Rainer says there are no odous to man­age be­cause the sludge is al­ready quite well sta­bilised when it is spread in the dry­ing hall, “by the time the sludge goes into the green­house it is al­ready at an ad­vanced stage of sta­bil­i­sa­tion”.

The new sys­tem is de­signed to han­dle 500 tonnes of dry solids a year. Right now it is han­dling the waste of 30,000 peo­ple, but the in­stal­la­tion has been fu­ture-proofed to meet the needs of 60,000 peo­ple. Ex­pand­ing the ca­pac­ity of the plant will es­sen­tially mean du­pli­cat­ing much of the equip­ment that’s been in­stalled.

Rainer says: “We are achiev­ing 90 per­cent dry solids, and every­one is pleased with how well the syetm is work­ing.”

Cur­rently, all the dry biosolids are be­ing stock­piled at the Kate Val­ley land­fill site near Christchurch. But there are plans to di­lute it with ‘green’ waste and spread it on land (sub­ject to re­source con­sent). Rainer says there is a huge po­ten­tial for this tech­nol­ogy in New Zealand.

“It’s rel­a­tively low-tech, low en­ergy, and makes use of the sun – it is sus­tain­able, en­vi­ron­men­tal­lyfriendly – it gets all the ‘green’ ticks,” he says. “One of the big­gest prob­lems in New Zealand at the mo­ment is that there are is a lot of sludge and ponds, and what do we do with the sludge?

“Be­cause the sludge con­tains so much wa­ter, when it comes to trans­port­ing it, you are re­ally mov­ing a lot of wa­ter (80 per­cent) and 20 per­cent solids. With the tech­nol­ogy we have used, the coun­cil ends up with just 10 per­cent wa­ter.

“There are lots of fa­cil­i­ties in the coun­try that could use this tech­nol­ogy, maybe 20 to 30, there are small com­mu­ni­ties and dairy waste too.”

The dry­ing hall for biosolids has pro­vided a cost sav­ing of about $3 mil­lion to $4 mil­lion for Sel­wyn coun­cil by re­duc­ing the sludge des­tined for the land­fill.

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