As Close to Drinking Water as it Gets: A Model Case Study of Stormwater Treatment, Compliance and Success

Metalcorp NZ Ltd, established in 1989, operates as a family-owned business from two branches within the South Island of New Zealand. The primary function at Metalcorp is the collection and processing of ferrous and non-ferrous metals into the raw materials that other industries require to produce new products for both domestic and international markets. As long-term members of the New Zealand Association of Metal Recyclers, Metalcorp has demonstrated their commitment to raising the environmental protection standards within the metal recycling industry by proactively installing a series of stormwater treatment BMPs at their 0.6 hectare (1.5 acres) facility.

Monitoring action by the Christchurch City Council indicated that the facility exceeded trigger levels for stormwater discharge of aluminum, copper, zinc, lead and magnesium. The trigger levels for Metalcorp were based on local regulations which sets the levels required as more pure than drinking water – levels unnecessarily stringent for stormwater runoff. Despite being subject to this exceedingly high bar for their stormwater quality, Metalcorp decided to proactively implement BMPs to bring the pollutant discharge levels down closer to trigger levels. Not finding satisfactory solutions within New Zealand, Metalcorp connected with StormwateRx personnel at a scrap recycling conference in the United States. In 2019, Metalcorp installed a Clara 25C up stream of an Aquip 110SBE for their Christchurch facility.

After a year of running the StormwateRx units, Metalcorp collected samples and ran tests, and submitted the results to Stormwater Audits, a branch of New Zealand’s governing entity Christchurch City Council. The results demonstrated that Clara effectively removed bulk solids and metals with Aquip further polishing the effluent. These BMPs together brought TSS, aluminium, cadmium, and chromium below the trigger levels while also significantly reducing copper, lead, and zinc to less than six times the trigger levels. Based on the results which assume 0.54 ha (1.3 acres) for C1-C3 and 650mm (26 inches) average rainfall for Christchurch, the Clara and Aquip are mitigating an estimated 555 kg (1224 lbs) COD, 2 tons of solids, 16 kg (35 lbs) aluminium, 2 kg (4.4 lbs) each of copper and lead, and 50 kg (110 lbs) of zinc per year from Halswell Junction Outfall, the watershed most affected by runoff from the Metalcorp facility. As the Council Advisor put it: “Not quite drinking water quality, but nearly!”

Metalcorp Sampling Results

ParameterUnitStormwater Trigger Value09/07/20 Raw09/07/20 After Clara09/07/20 After AquipEstimated Mitigation Impact (kg/yr)
Duration since last rain event (>3mm)day-111
pHSU6.5-8.58.07.77.1
Chemical oxygen demandmg/L1201807622555
Solids-Total Suspendedmg/L50590594.22056
Total hardness (as CaCO3)mg/L44
Aluminium acid extractable mg/L0.084.51.5<0.2015.8
Cadmium acid extractable mg/L0.00040.00680.0017<0.00170.02
Chromium acid extractable mg/L0.0060.0420.00930.00120.14
Copper acid extractablemg/L0.00180.00180.120.00862.15
Lead acid extractable mg/L0.00560.00560.150.0112.52
Nickel acid extractable mg/L0.0130.0130.062<0.00100.22
Nickel acid extractable mg/L0.0150.01514.00.08848.8

“Hi Guys! We received the results in today from our first Council testing of our stormwater since everything was installed. These are GREAT results! We couldn’t be happier with the system. It’s superb! Dare I say worth every penny. Cheers and HUGE THANKS.”

~ Korina Kirk