How much plastic can be saved by using cloth bags in 1 city?

Answer 1

First it is necessary define all requisite variables. Thus, we must define shopping rate, and city.

There are many ways to define a city, but for our purposes perhaps we should define it in terms of population density. Given the set of the 48 most populous cities with known population density (http://en.wikipedia.org/wiki/List_of_cities_by_population); we can calculate the average to be approximately 9290 people per square kilometer of city (9290 person/km2).

We can attempt to define the shopping rate by starting with some assumptions about shopping. Let's start by assuming that our discussion is limited to groceries; thus, we will only count plastic bags that are used for carrying things that people will eat. Next, we'll assume that the volume of the food product has the same value as the total volume of the item; in essence: the packaging of the item has zero volume. Third, let us assume that a person consumes exactly one stomach full of food per day and that all of the food is that which they purchased from the grocery. Additionally, we will assume that the food packs perfectly in each bag and that a plastic bag is always used for groceries. If we also assume that a human stomach has a 1.5 liter capacity, and that a typical plastic shopping bag has a capacity of approximately 7.5 liters, then we can define shopping rate in the following manner: divide the rate of consumption (1.5 liters/day-person) by the capacity of the shopping bag (7.5 liters/bag) to yield a rate (0.20 bags/day-person).

Finally, now that we have the population density and rate per person each day we can multiply them to get the daily bag density: number of bags per day for each square kilometer of the city (1858 bags/km2-day).

So, under these assumptions approximately 1858 bags can be saved per day for every square kilometer of a city if the plastic bags are replaced by cloth.

We can make a few more assumptions with this result; if we use the area data for the set of 48 cities we can get an average area of 1485 km2 for a city. Thus, each city uses 2759130 bags per day. That's 19313910 bags per week and 1007082450 bags per year.

The mass of a plastic shopping bag is approximately 4.5 g, so the plastic saved by replacing all bags in the one city we defined is approximately: 613140 grams per day 4291980 grams per week and 223796100 grams per year. (613 kg/day; 4292 kg/week; 223796 kg/year)

This is only for one very populated city; if we apply this switch from plastic to cloth to each of the 48 cities used in our calculations we save: 29431 kg/day; 206015 kg/week; 10742213 kg/year. That's over 10 million kilograms of plastic per year saved for the 48 of the most populous cities in the world!

An interesting side note: with LDPE (the plastic most common in shopping bags) at 0.25 USD per kg, that's more than 2.5 million dollars worth of plastic.

Keep in mind the important assumptions we used; this number makes very generous assumptions about the usage of bags. We assume that plastic bags are always being used to their absolute maximum capacity, that plastic bags are only used for food items, that the entire volume of the item purchased is eaten, and that all people are purchasing their food and putting it in plastic bags. It does not account for bags not packing to 100% capacity, bags used for non-food items, and the fact that people can eat without having to visit a grocery store. So, the estimation is almost certainly not accurate; it would not be surprising if this is an underestimation since: the best packing of random non-regular polyhedra and ellipsoids is probably no greater than 75%; in reality, packaging tends to consume a large portion of a products and a large volume of an item; and, of course, a very large volume of non-food items are purchased, which also fill plastic bags.