Wednesday, May 16, 2018

Human Waste Treatment compared to Livestock Manure Management


A while back I wrote about why human and animal waste are treated and managed differently. In many respects, this was an economic rationalization why we chose to do things so differently with the same goal in mind, protection of water quality. The foundation behind it was right, but it stopped short of the question I get more often, which one is more effective.

While it’s important to keep in mind that there are good reasons to manage them differently, I’m going to make a simple comparison between the two systems. Granted there is a lot more we could focus on than just BOD (biological oxygen demand, the amount of oxygen needed to break down the waste which tells us something about the risk of a fish kill) and the amount and forms of nitrogen that comes out of each system.

To recap what was covered in the first post, there are significant differences in how human and animal wastes are managed.  Human waste (assumed here to be domestic waste only, no industrial in this post) is typically treated and discharged to receiving waters.  Animal manures are typically stored and land applied.

Several factors influence the difference in approaches including:
      • Wastewater characteristics
      • Regulation
      • Economics
The following table compares typical waste characteristics and volumes for a 10,000 population city) and a 10,000 head hog farm, for both total volumes produced and the characteristics of it.

Table 1 – Human and Animal Waste Comparison (1,2)
Parameter
10,000 Population City
10,000 Head Hog Farm
Value
Assumption
Value
Assumption
Volume
1,250,000 GPD

456.3 MG/yr
125 gpd/capita
12,000 gpd
4.4 MG/yr
1.2 gpd/head
BOD
1,900 lb/d
693,500 lb/yr
0.19 lb/d/capita
3,030 lb/d
1,105,500 lb/yr
30,350 mg/L
Nitrogen
300 lb/d
109,500 lb/yr
0.03 lb/d/capita

700 lb/d
255,000 lb/yr
7,000 mg/L

Phosphorus
80 lb/day
29,200 lb/ yr
0.008 lb/d/capita
95 lb/day
76,500 lb/yr
2,100 mg/L





Oxygen Demand
3,470 lb/day
1.1 lb O2 per lb BOD and
4.6 lb O2 per lb nitrogen
6,550 lb/day

1.1 lb O2 per lb BOD and
4.6 lb O2 per lb nitrogen

Regulation and Estimated Nutrient Loss

All wastewater treatment plants (WWTPs) must meet the requirements of their discharge permit as part of the Clean Water Act.  Typical permits include limits for BOD, solids, ammonia, pH, and disinfection to kill pathogens.  Focus on the harmful effects of nutrients (total nitrogen and phosphorus) in watersheds (depressed oxygen levels, algal blooms, fish kills) has led to increased regulation of nutrients for many treatment plants, often requiring advanced and expensive treatment processes.  Most treatment plants land apply treated solids and must meet regulations with limits on pathogens, nutrient loading rates and application practices. 

While this definition of the process is helpful, what we typically want to know is how many pounds of BOD, N, and P we are allowed to discharge per person. That is what is the actual effect we have?  Looking at the city of Ames wastewater permit, our municipal treatment facility is allowed 2018 lb/day of BOD, and 284 lb D of NH3-N lb of N per day. For fun let’s say Ames has a population of 66,000. This is 11 lb BOD/per person per year and 1.6 lb NH3-N/person per year discharged.  They don’t mention nitrate, but it’s probably about 9.4 lbs NO3-N as very little is denitrified using the current technology they have (though this is subject to change).

Most animal operations, once they hit a size threshold of 1000 animal units, are subject to an NPDES permit if they propose to discharge. Iowa law actually doesn’t allow confinement farms to discharge, so on the point source side the regulations are pretty stringent and the number would be zero except in extreme weather conditions. However, land application of animal manures is an important part of nutrient transport. Let’s work off a pig space, so at 1.2 gallons per day and N content of 60 lb N per 1000 gallons of manure. This works out to about 30 lbs of N per pig space per year, so about 0.2 acres fertilized with the manure. We lose about 30 lb N per hectare as nitrate leaching when we grow row crops, so we are losing about 5 lb N per pig space per year, so about half of what we lose per person. Losses of NH3-N in water and BOD in water are very minimal do to the effective treatment of soil.
Figure 1. Water quality is important to all Iowans. Different treatment approach can both help achieve desired water quality objectives.

Final Thoughts

1.      If we didn’t recycle manure as a fertilizer would the nutrient load to streams increase, decrease, or stay the same?
2.      If we say there are no point source losses from collection to storage (a mostly true assumption) how much does manure contribute to the nutrient loading?

If we treated manure like municipal waste, the nitrogen loss actually goes up as we just replace the manure with other synthetic fertilizer (meaning non-point source losses stay similar, though some change is possible). We’d also have additional nutrient loss; though we may remove most of the BOD and almost all the ammonia in the manure, we’d still have nitrate released from the treatment plant into our streams and rivers.

The other interesting take away was that while the two approaches for treatment were drastically different, they both seemed to be equally effective at removing BOD and ammonia from water, but have some difficulty with nitrate though losses per pig space are estimated to be about half of that from a human.