Treated Landfill Leachate - Impacted Stormwater With A Nitrifying Trickling Filter - Not a Load of Rubbish

Liam Allan (Pattle Delamore Partners Ltd), Dr Mark Ellis (Pattle Delamore Partners Ltd), and Dr Sachin Narkhede (Timaru District Council)

Timaru District Council (TDC) has operated the Redruth Landfill, on the southern outskirts of Timaru, since the 1900s. Historic landfill cell designs and management practices did not fully comprehend that leachate would be generated in the landfill and seep from the landfill cells into stormwater drains and adjacent water bodies that bound the landfill. This, in turn, has negatively impacted the aquatic ecology of the lower stretch of the Ōtipua-Saltwater Creek. Improving the management of leachate and stormwater management practises at the landfill has been an ongoing task for TDC, specifically developing and implementing solutions to minimise the adverse effects on the aquatic ecosystem of the lower Ōtipua-Saltwater Creek. Timaru District Council have a vision of ultimately returning the creek into a valued cultural and recreational asset for the community. 

To mitigate the adverse effects of ammonia in leachate-impacted stormwater, TDC engaged Pattle Delamore Partners Ltd (PDP) to identify and evaluate possible treatment solutions that could (1) mitigate the effects on the aquatic ecology, (2) required minimal supervision and maintenance, and (3) had a low capital or operational cost. The solution ultimately selected was to construct a nitrifying trickling filter (NTF) on the embankment of a historic landfill cell and the bank of a stormwater pond.

The design developed used locally sourced rock that provided a medium for the treatment biofilm to adhere onto. The structure of the NTF was achieved by constructing the walls from gabion baskets on three sides of the filter. The net result was an aesthetically pleasing structure that was readily visible from a nearby public walkway.

The NTF was constructed and commissioned in September 2020 and has routinely achieved a 93% reduction in total ammoniacal nitrogen concentration, resulting in an average treated total ammoniacal nitrogen concentration of 0.55 g/m3. In addition, the NTF contributed to a 16% reduction in the total nitrogen concentration in the influent stormwater.

The discharge of treated stormwater into the downstream stormwater pond resulted in an unforeseen and beneficial benefit, specifically a noticeable improvement in the clarity of the pond, which has been attributed to the release
natural coagulants, which has been attributed to extracellular polymeric substances in the NTF biofilms. 

This paper demonstrates how the technology normally associated with the treatment of wastewater can be re-engineered and optimised to manage the impacts of elevated ammonia in leachate contaminated stormwater.