People, Pigs, and Poop

 

Recently, there was a little exercise for how much swine poop there is in Iowa and turning it into a pyramid. In that exercise, they (Raygun – I didn’t fall out of my chair or roll my eyes. I was excited, conversations about manure are welcomed) calculated about 85 billion pounds of pig poop per year – I won’t dispute that number though I calculate a slightly higher amount. I’ll even add in cattle and poultry and estimate 85 million tons of livestock poop annually.

But how much human poop is there? Iowa has about 3.2 million people in it. A person poops about 175 grams per day or 0.38 lbs. This is about what was assumed when they compared humans to pigs, but unfortunately, that’s not the pig number they were using; the pig number includes urine and wash water, too. As it should, because, in the name of water quality and manure management, we would also manage that component. Humans generally make about 0.37 gallons of urine daily, so another 3 pounds of material. So, humans are up to 3.4 lbs of “manure” a day, not that 0.38 lbs.

I mean, if we are going to talk “poop,” we probably want an apples-to-apples, or a poop-to-poop comparison, don’t we? But here is where it gets complicated – for my livestock manure numbers, I include wash water volumes – because we manage it like manure, and we should view this wash water like manure. I’m glad we do. But does that mean for humans we should include our “wash water” as well? That would include the water you use when you flush a toilet, shower, or do dishes. In developed countries like the US, the average person generates about 80-100 gallons daily. Let’s go with 90 gallons a day, or 750 pounds a day. Extrapolate this to a year, and you get 438 million tons of human wastewater! Or about five times what we generate from livestock production.

So, let’s play the game of how much poop is there?

So, for a pig, we have about 10 pounds a day, about 10% of this is solids material, and I’m going to say that fecal material is about 50% moisture, so a pig excretes about 2 pounds of feces a day, 8 pounds of urine. Throw in wash water used at the site, and we are at 10.8 lb/day. So, what’s the comparison now?

Table 1. Comparison of human and pig related “manure” and wastewater generation.

	Human	Pig
Feces	0.38	2	lb/day
Poop (urine + feces)	3.4	10	lb/day
Wastewater	750	10.8	lb/day
Population	3,200,000	30,500,000	million
Feces	221,920	11,132,500	tons/yr
Poop (urine + feces)	1,985,600	55,662,500	tons/yr
Wastewater	438,000,000	60,115,500	tons/yr

 How we choose to manage wastewater greatly influences the question of what characteristics are important for me to know about that wastewater. Alternatively, the characteristics of the wastewater greatly affect how I’d choose to manage the wastewater. All that to say, a simple volume comparison isn’t enough; we have to dig deeper. What does this tell us? Humans send more “manure” to wastewater treatment systems in Iowa than livestock would, but the animal manure would have more feces in it. But this, at its heart, is why we choose to manage human and livestock manures so differently. If you have a lot of water, not much stuff in it, and are far from cropland, treatment and discharge makes sense. If you are managing volume to be smaller and get higher nutrient concentrations in it, then making decisions to use that material to replace fertilizer makes more sense.

So, how should we think about wastewater and characterize it? There is more to it than this, but if we want to keep it simple, we should start with four parameters.

Total volume, chemical oxygen demand (COD), nitrogen, and phosphorus. Why these four? Because, at their core, they tell me a lot about how poop could impact the environment. How much are we dealing with, what’s the immediate impact to water (chemical oxygen demand), and what is the potential for eutrophication (nitrogen and phosphorus).

Alright, let’s look at chemical oxygen demand. For untreated municipal wastewater the COD/five-day biochemical oxygen demand (BOD5) ratio is about 2. Why am I using this ratio? BOD5 is a much more common measure of wastewater strength (great history to this measurement, and it comes from London and the Thames River – basically because it took five days for the sewage they dumped in the river to make it to the ocean). So, what’s the BOD5 of municipal wastewater? It depends, but a good average number is around 220 mg/L. I’ve also included N and P in human wastewater and what I estimate is excreted by a pig for comparison (Table 2).

Table 2. Estimated COD, N, and P in human and swine wastewaters.

	Human	Pig
Wastewater	750	10.8	lb/day
COD Concentration	440	84,500	mg/L
N Concentration	40	8400	mg/L
P Concentration	8	1360	mg/L
COD Mass	192,165	5,065,142	tons/yr
N Mass	17,470	503,517	tons/yr
P Mass	3,494	81,522	tons/yr

 How do we try to turn this into water quality impacts? Quantifying impacts is difficult, it requires us to make assumptions about how treatment and utilization impacts COD, N, and P movement and losses to water quality. With municipal wastewater, we typically treat and then discharge. To quantify what may be making it to a stream, we have to estimate the percent removal with treatment and then quantify where it ends up. For COD, hopefully, around 90% will be removed, and this will be mostly converting material into CO2 (70%) and municipal solids (20%).  The municipal solids would then be land applied. However, land application is highly effective at COD removal and preventing it from entering water, so we’ll say 0.05% is lost from the land applied fraction.

In terms of nutrients, it gets a little more complicated and depends on the treatment system being used. For phosphorus, hopefully 50% of the P ends up in the municipal solids (which are land applied) and 50% are discharged after treatment. Of those land applied, again it depends on the management practices used, but assuming good phosphorus management, probably only 0.5% of the P land applied moves with water from the land application area. For animal manures, we will use the 0.5% for all phosphorus as it should all be land applied. In the case of nitrogen, ultimate fate is again harder because it is very much dependent on if the wastewater treatment method employed. Still, for a working version of what is happening, we’ll go with 30% is denitrified, 40% is nitrified and discharged, and 30% is recovered in the wastewater sludge and land applied (assume 20% of N is lost during storage before land application). Assuming that it is land applied as a fertilizer, we’ll go with 20% of the nitrogen is lost after land application. With manure, I’m going to assume 20% is volatilized during manure storage and lost to the environment and that, again, 20% of the nitrogen that is land applied is lost. I’m providing these results in Table 3 to show an estimated N loss.

Table 3. Estimated impact on the environment from human and pig manure after treatment for human wastewater and land application as a fertilizer for pig manure.

	Human	Pig
COD	19,236	2,533	ton/yr
N	8,875	181,266	ton/yr
P	1,756	408	ton/yr

 Where does that leave us? Swine manure probably is having more impact on the environment than human wastewater in Iowa. At least in part this is due to the vast differences in populations of pigs and people. I’ll give you one more table, COD, N, and P estimated to be released to the environment, but on a per person and per pig basis.

Table 4. Estimated impact on the environment per person or per pig after treatment for human wastewater and after land application as a fertilizer for pig manure.

	Human	Pig
COD	12	0.2	lb/person(pig space)-year
N	6	12	lb/person(pig space)-year
P	1	0.03	lb/person(pig space)-year

 I want to do this one more time (table 5). What happens if we say that where we were applying manure would have received fertilizer anyway. Well, assuming the manure is being managed like a fertilizer, nitrogen and phosphorus losses from that acre would be similar. That is, the losses are driven by land use, and not directly by manure (and we can, and should in the future have a discussion on if manure is being managed as well as commercial fertilizer, and how to continue to improve our management of both).

Table 4. Estimated impact on the environment per person or per pig after treatment for human wastewater and after land application as a fertilizer for pig manure.

	Human	Pig
COD	12	0.2	lb/person(pig space)-year
N	5	12	lb/person(pig space)-year
P	1	0.03	lb/person(pig space)-year

 Each method, municipal treatment for human wastewater and storage and land application of manure for livestock, has its pros and cons. If we were to treat pig manure like human waste, does water quality get better? For COD and P, I don’t think so; in fact, we probably add more of each to Iowa water ways using this method. If we treat N like human waste – it’s complicated and depends greatly on the amount of N that goes into denitrification, but unless there was a land use change associated with no longer having manure as fertilizer, we’d still get some of the losses with the use of commercial fertilizer on those crop acres that we get right now when we use manure.

The system is complicated. We need to continue to innovate to reduce N volatilization losses from storage. Specifically, these volatilization losses, are what make nitrogen losses from manure greater per pig than per person. We need to continue to develop improved nitrogen utilization practices and nitrogen fertilizer recommendations tailored to each year, location, and growing season so we can do better utilizing manure nutrients and lessen impact on water quality. The conversation is difficult, and hopefully, that comes through, that it is more than a pyramid of poop.