Why is human waste treated and discharged and animal manure stored and land applied?

As you can tell from the title, I've branched out and decided to hit a topic that is a bit different than some of the others we’ve touched on. What I’m going to discuss is why we handle human waste and animal manures so differently. This is a fun topic with lots of nuances and unfortunately we won’t be able to hit on all of them, but as this is a pretty common question, i.e., why don’t our animal farms just treat their manures like cities do, I think its work talking about.

As you are probably aware, town and cities use sewage treatment facilities to process human wastes. The treatment process create a purified (at least to some level) liquid fraction that is usually discharged to surface water and often times, a solids rich portion that is land applied on farm fields. The approach on livestock farms is very different, in this case the manure is stored and then applied to crop ground as a fertilizer.

So let's cut to the chase, why is human waste treated and discharged and animal manure stored and land applied? There are a few differences and I’ll try to hit on them, but the big one is WATER consumption! There are lots of things in both animal manure and human waste that can impact water quality… nitrogen, phosphorus, potassium, solids, and even pathogens…, but one big difference is how much water is used, and as a result how much manure/waste is generated.

You’ll often hear things like a farm of 10,000 finishing pigs will produce as much nitrogen of phosphorus as a city of 46,000 people, but it only takes about 550 people to use as much water as those 10,000 pigs! Why does this matter… well, it means our animal manures are a lot more concentrated than human waste! This means it doesn’t take as much to cause and environmental issues and that conventional waste treatment systems used for human waste often won’t perform well when used for animal waste. However, just as importantly the lower water use at swine farms makes it much, much more feasible and economical to collect, store, and apply it to our crop fields as a fertilizer.

Let’s play with a few numbers on this to get some idea about the economics; with swine manure it probably costs about $0.02 per gallon to land apply. My math says that 10,000 pigs will make about 4.4 million gallons of manure per year, meaning our manure application cost would be about $88,000, but this swine manure contains about $200,000 worth of fertilizer value based on the nitrogen, phosphorus, and potassium it contains, making our storage and land application approach a very viable utilization strategy. So what about our human waste? Well in this case since the manure is much more dilute our costs to land apply it are probably a bit cheaper, let’s say $0.005 a gallon. In this case a city of only 550 people with generate the same about of water so it would cost $22,000 to land apply, but it would only contains a little more than 1% of the nutrient value  of the swine manure, so it only has a fertilizer value of around $2,500. So in this case this doesn’t seem like a very good approach to managing our human waste since the cost to handle it is more than the fertilizer value it would provide.

In large part, this is the fundamental difference. The reason we store and land apply our animal manures is because they are concentrated enough that often times utilizing the manure as a fertilizer source is economical. In the case of human wastes, this isn’t the case because of how much water we use. Several other factors also play a role, but at the fundamental level, it really comes down to water use and our ability to economically use it as a fertilizer.