Which milk container is greener?

Glass is mostly made from sand, which we have lots of in NZ. The process of making glass, however, is very energy intensive and requires heating the raw materials in a furnace held at over 1500°C.

Glass bottles are heavy with glass typically being 40 per cent of the total weight when filled with milk (compared to 2 per cent for bottles and 3 per cent for cartons), meaning lots of energy is used for transport. They were designed to be delivered daily so only carried small volumes of milk (600ml) and had relatively short shelf lives compared to other containers because the glass let in so much light, which spoils milk.

The advantages of glass come into play when they are reused, as was commonplace through the home delivery milk service. With a simple sterilization procedure, the bottles could be refilled several times before being recycled which made them very energy efficient. Sadly, without a national glass bottle return service, they are now typically sent straight to recycling which again involves high temperature processing and is very energy intensive.

Both the standard and light-proof bottles are made from high density polyethylene (HDPE). The raw material is ethylene gas, formed by heating naphtha from crude oil and then processed into pellets which are later shaped into bottles. Crude oil is a non-renewable resource.

The light-proof bottles include layers of carbon black and titanium dioxide to protect the milk from UV damage and to colour the bottle white. Carbon black is made by burning petroleum oil and collecting the carbon black particles, and titanium dioxide is mined from ilmenite. Neither of these processes is particularly environmentally friendly, and these are non-renewable resources.

Only 2 per cent of the total weight of a filled bottle is from the plastic which means less energy (from petrol or diesel) is needed for transport.

Milk maintains its vitamin content for longer and stays fresher, resulting in less milk wastage.

Light-proof bottles can be recycled but have a lower monetary value in the recycled plastics trade compared to normal bottles due to the added colour, which limits products they can be recycled into.

Both the standard and light proof bottles are made from high density polyethylene (HDPE). The raw material is ethylene gas, formed by heating naphtha from crude oil and then processed into pellets which are later shaped into bottles. Crude oil is a non-renewable resource.

The life cycle of the bottles is pretty good as they are easy to recycle and picked up through curb-side collection. However, New Zealand law prevents recycled plastic from coming into contact with food which means your milk bottle won’t be recycled into another milk bottle, but instead into products such as plastic furniture, buckets, crates, and office stationery.

Only 2 per cent of the total weight of a filled bottle is from the plastic which means less energy (from petrol or diesel) is needed for transport.

Making regular plastic milk bottles releases 39 per cent less greenhouse gas than making glass bottles, and 24 per cent less than cartons.

Wood pulp to make the cardboard is from a renewable source and while growing, the trees absorb carbon dioxide from the environment.

Cardboard alone is weak (recycled cardboard is weaker) and not waterproof, so the paperboard in Tetra Pak cartons must be brand new and coated with a plastic called low density polyethylene (LDPE) to be waterproof. Because the porous paperboard soaks up the plastic, it makes them very difficult to separate again upon recycling. This is a big problem as currently only recycling depots in Auckland and Christchurch will accept Tetra Pak cartons. Therefore, a lot of energy is used making cartons from brand new materials which are then sent straight to landfill.

Making Tetra Paks releases 18 per cent less greenhouse gas than making glass bottles.

Only 3 per cent of the total weight of a filled carton is from the cardboard which means less energy (from petrol or diesel) is needed for transport.