Solids Separation in Dairy Manure: Digging Deeper into the Tools for Phosphorus Control

 Is solid separation needed to help you manage phosphorus accumulation in soils in your field? Dairy manure tends to have more phosphorus than most cropping systems need when we apply nitrogen-based rates. Over time, this leads to nutrient loading, P Index concerns, and tougher decisions about where to haul your manure.

My last article discussed strategies for tackling this imbalance, like targeting low-P fields or adopting practices that hold soil in place. But what if your fields are already saturated, your neighbors aren't lining up to take solids, and you're still trying to manage more manure than you have places to put it?

Let's dig into solids separation—not as a silver bullet, but as a tool that gives you more control over where your nutrients go. We'll focus on the nuts and bolts of how these systems work, what kind of phosphorus they capture, and what the real-world tradeoffs look like.

Why Separate?

The basic idea is this: when we separate manure, we split the nutrients into two different forms:

·         Solids, which are enriched in (especially the organic and particulate-bound forms)

·         Liquid, which contains most of the ammonium-N

This gives us flexibility. Suddenly, you're not stuck applying the full nutrient load to every acre. You can:

·         Haul solids to further fields without hauling water

·         Retain nitrogen-rich liquids for near-barn applications

Let's look at the tools we've got—and what they do regarding phosphorus removal and system compatibility.

Tools of the Trade: Separation Systems and Their P-Capturing Power

Slope Screen or Static Screen

The slope or static screen might be your entry point if you're looking for the simplest mechanical manure separator. These systems rely entirely on gravity—no moving parts, no motors, and minimal maintenance.

How It Works

Manure is pumped or flows across an inclined, slotted screen, usually mounted at a 15–30° angle. As the liquid manure flows over the screen, coarse solids catch on the surface and slide down while the liquid passes through into a collection tank or lagoon.

This method works best with thick, undiluted manure and a relatively high amount of large fiber.

Screw Press

The screw press is one of the most common and accessible solids separation systems on dairy farms today. It strikes a balance between simplicity, cost, and performance—making it a solid choice for farms looking to improve manure handling and pull out some phosphorus without diving into full-scale treatment systems.

How It Works

At its core, the screw press is a mechanical filter. Manure is pushed into a cylindrical housing where a rotating auger presses the material against a screen (often wedge wire or perforated steel). As pressure builds, the liquid is squeezed out through the screen, and the remaining solids are discharged at the end of the auger as a damp, fibrous cake.

The design is straightforward, with relatively low energy and maintenance requirements. No chemicals, no high-speed moving parts, and no need for complex control systems.

Centrifuge

On the other end of the solids separation spectrum, you've got the centrifuge—a high-speed, high-cost solution aimed at serious nutrient recovery. Centrifuges are more common in commercial or tightly regulated livestock operations, but they're becoming increasingly relevant where phosphorus management is mission-critical.

How It Works

A centrifuge spins manure at high speeds—often thousands of revolutions per minute—causing particles to separate by density. Heavier solids (and the phosphorus they carry) are flung to the outer wall and collected, while the lighter liquid stays near the center and exits through a different outlet.

Expected Performance

Separation performance was surveyed by Hjorth et al. (2009). The summarized performance is shown in Table 1. While all separators had the potential to alter the N:P ratio and create a solid fraction that could move phosphorus further from the barn, centrifugation had a much higher potential.

Table 1. Separator performance and variation in performance for solids, total nitrogen, and total phosphorus separation into the solid fraction.

	Separation Percent (% in solid fraction) ave. (s.d.)
	Dry Matter	Total N	Total P
Sloped Screens	47 (28)	34 (28)	30 (22)
Screw Press	34 (15)	15 (12)	13 (7)
Centrifuge	63 (15)	36 (14)	69 (17)

 

Table 2. Estimated characteristics of the separated solids and the remaining liquid after separation. Centrifugation has the potential to substantially alter the N-to-P ratio of both the liquid and solid fractions.

	Solids Characteristics (lb/ton)				Liquid Characteristics (lb/1000 gallons)
	Solids (%)	Total N	P2O5	Available N:P	Solids (%)	Total N	P2O5	Available N:P
Sloped Screens	14	8	3	1.3	4	16	6	1.9
Screw Press	23	8	3	1.3	4	18	7	1.9
Centrifuge	33	16	11	0.6	2	14	2	4

 

Thinking Ahead: What's Separation For?

Solid separation doesn't eliminate your phosphorus problem—it reshapes it. It works best when it:

·         It buys you the flexibility to put nutrients where they're needed

·         Enables cost-effective solids hauling to further fields or sell to other farmers.

Final Thoughts

Solid separation isn't magic, and it won't make phosphorus disappear. What it can do is give you options. It lets you make smarter nutrient management decisions, especially when your fields are overloaded, or your P Index is getting tight.

·         Screens and screw presses are relatively affordable and easy to operate—but don't expect miracles.

·         Centrifuges can remove a lot more phosphorus, but they come with a big price tag and more management.

·         None of them solve the problem unless you also solve the logistics of where those solids go.

For most dairy farms, separation makes the most sense when it's part of a bigger strategy:

·         Matching nutrients to fields that can use them

·         Using agronomic practices to reduce runoff and improve soil uptake

·         Thinking ahead about storage, labor, and timing

And if you're not ready to invest in new hardware? You can still manage phosphorus smartly by focusing on P Index risk reduction. Cover crops, ensuring erosion control, using thoughtful application timing, and squeezing every bushel of uptake you can out of your crop system.

Solid separation isn't a silver bullet. But it can be a good wrench in the toolbox—especially when you know what job you're trying to get done.