Category : Water Treatment
The North Waikato Regional Landfill at Hampton Downs is New Zealand’s largest landfill. Situated between Auckland and Hamilton it receives 500,000 Tonnes per annum of municipal waste from Auckland.
The landfill needs to dispose of 150kL to 300kL of leachate each day. This was being transported offsite and discharged into the Auckland sewer 70 km away.
How Much it was Costing Them
To transport and discharge the leachate cost $22 /kL, or between $3,300 – $6,600 a day. The continued tankering of leachate offsite for the landfill’s anticipated life of 35+ years was quite literally throwing atleast a million dollars a year down the drain.
The owner and operator of the landfill, EnviroWaste Services, projected that as the size of the landfill grows this cost will increase by a further one million dollars every five years. By 2020 the transport and treatment costs alone will be $2.5M. Add to this the capital costs of more storage tanks and tanker trucks, price increases by the waste water treatment plant and the ever present risk they could change their acceptance criteria.
What needed to change
The ability to treat and dispose of the leachate onsite was desired. To achieve this the processes considered included:
- biological and chemical treatment
- reverse osmosis
Evaporation was dismissed as nonviable. Evaporation to air is not possible due to Auckland’s average relative humidity in the range of 70% – 85%.
The landfill generates and harvest its own landfill gas, which it uses to generate electricity that it can on-sell. Using this gas to power an evaporator was investigated but the cost in lost revenue from electricity sales would be $38 per kL of leachate treated.
If the leachate is not vaporised and released into the air then any liquid or solid effluent/by-product of the treatment process would need to be disposed of to land or a waterway. Solid or sludge by-products could be returned to the landfill but any liquid effluent that could not be reused on site would need to be irrigated or released into a waterway. Their consent to irrigate to land only permits this when there is a soil moisture deficiency and only allows 150 kL per hectare per three days, and only from October to March. The Waikato River runs just east of the landfill and a consent to discharge into the river was granted, but with strict water quality criteria, particularly regarding Boron and Ammonia, and an allowable conductivity of only 20 μS/cm.
Reverse osmosis was going to be needed in order to achieve the strict environmental consent quality requirements. Traditional spiral wound reverse osmosis membranes would require pre-treatment, using either a biological system or otherwise a series of micro and ultrafiltration membranes, in order to prevent them from fouling and constantly needing cleaning.
Alternatively a single pass through a vibrating membrane was proven in a series of pilot tests to adequately clean the leachate without fouling the membrane. The principle behind the vibrating membrane is that the vibration prevents the membrane from fouling.
Syngineering designed and constructed a treatment plant capable of treating 300 kL/day. The system uses six VSEP systems to reduce the feed water with a Total Dissolved Solids (TDS) of 5,ooo ppm down to 550 ppm.
The permeate from the first stage is then treated through four stages of spiral RO.
Through this process it is possible to achieve a final permeate that is well below the required levels:
- TDS of 0.1 ppm, down from 5,500 ppm
- COD of 0.002 mg/L, down form 4,100 mg/L
- Ammonium of 0.08 mg/L, down from 930 mg/L
- Boron of 0.18 mg/L, down from 26 mg/L