Wet oxidation :
- much faster processing technique,
- has a reduced environmental impact with fewer greenhouse gas emissions,
- has a higher technology readiness level (TRL)
- it also can significantly reduce the solid content for easy transport and landfilling (Hii et al., 2014a). This hydrothermal treatment also has significant advantages over thermal processing methods including reduced pre-treatment and separation requirements as well as wet oxidation being a less energy intensive process. Wet oxidation can help recover resources from waste (Munir et al., 2018). Resource recovery from waste is a fundamental change in approach from waste treatment alone to waste treatment with resource recovery also called the circular economy paradigm(Mansouri et al., 2017; Udugama et al., 2017).

Currently, the most common waste management technique is landfilling the municipal solid waste, both globally and in New Zealand. However, this is becoming increasingly expensive due to site costs and many sites being unable to accommodate the large volumes of waste being produced. Once waste is landfilled, there is little further opportunity for resource recovery. Furthermore, landfilled waste releases toxins, leachate, and greenhouse gas emissions. Apart from the financial cost involved, lack of resource recovery and toxic emissions associated with landfilling, the waste breaks down slowly (Lou and Nair, 2009).

To achieve financial solid waste management, we need to systematically analyse the strengths and weaknesses of the community as well as the municipal corporation based on which an effective decentralized system. The public can be altered by awareness building campaigns and educational measure. 

The core issue discussed in the manuscript is :
- To reduce solid content
- To recover value from municipal solid waste materials. 
Municipal solid waste such as non-recyclable plastics, treated timber, tyres, non-recyclable cardboards, and sanitary items (i.e. nappies) represent a major portion of ‘non-organic’ solid waste in New Zealand and globally.

Municipal solid waste are difficult to treat before their disposal and cause adverse environmental impacts when treated using biological or thermal processes. Treated timber is difficult to dispose of using landfill or alternate techniques because heavy metals can leach into the surrounding environment (Boyle, 2000). This issue is also relevant globally (Wang et al., 2017). Rubber tyres have a large amount of embodied energy, take a longtime to degrade, take up a lot of landfill space due to void spaces and can harbour insects and other vermin to spread diseases, heavy metals in tyres can leach into the environment, they can catch fire which is difficult to extinguish and releases toxic materials (Ministry for the Environment, 2006). Nappies are a significant component of the sanitary waste stream, methane emissions and leaching to groundwater may cause risks to human health.

Laboratory scale batch hydrothermal deconstruction tests were carried out under subcritical wet oxidation conditions. Treatments were carried out at a temperature range from 240 to 280 °C with initial pressure of 35 bar oxygen. Hydrothermal wet oxidation of waste is an emerging technique to sustainably treat waste with the common aim of waste solid reduction. During wet oxidation, waste is degraded in water under subcritical conditions of temperature (150 to 320°C) and pressure (20 to 150 bar). A hydrothermal vessel is pressurised using oxygen to maintain its contents in a liquid state. Waste is deconstructed and solubilised during oxidative hydrothermal wet oxidation (Muniret al., 2017).

 

There are several other waste management techniques available such as biological and thermal processing of waste. During biological processing, microbes degrade organic compounds in solid waste aerobically or anaerobically. Biological processing of municipal solid waste requires a large area and is time consuming and expensive. Some materials, particularly plastics require pretreatment before being biologically degraded (Eckenfelder and O’Connor, 2013). Thermal processing includes several further techniques such as incineration, gasification and pyrolysis, which involve high temperature degradation of materials. Many non-hazardous wastes can be treated using thermal processing, however, this results in greenhouse gas emissions and carcinogenic volatile organics (Lu et al., 2011). In addition to this, thermal processing requires the drying of waste as a pre-treatment step, which is a highly energy intensive and costly process.

The municipal solid waste sent to landfill in New Zealand contains : 
- organics (28%), 
- rubble (16%),
-  non-recyclable plastics (8%),
 - treated timber (11%), 
- glass (2%), 
- rubber tyres (2%), 
- sanitary items (3%), 
- non-recyclable cardboards (7%), 
- hazardous materials (15%) 
- other solids (10%) 
(Ministry for the Environment, 2009).