Lagoon sludge removal: what to know before you dredge

Lagoon sludge is the settled layer of manure solids, dead bacteria, sand, and grit that slowly fills a lagoon's treatment volume. Measure it with a sludge survey before you hire anyone. NRCS calls for removal once sludge takes more than half the treatment volume. Removal options are agitate and pump, draw down and excavate, or dredge into dewatering bags. Phosphorus in the sludge usually decides how many acres you need.

What is lagoon sludge, and why does it keep building?

An anaerobic lagoon is built to digest manure, but it never digests all of it. NC State Extension describes sludge as a mix of settled manure solids, the parts of manure bacteria cannot break down, live and dead microbial cells, and the sand and debris that wash in from the barns (NC State AG-639). That layer grows every year the lagoon runs.

How fast depends on what goes in. NC State's lagoon closure guide lists sludge accumulation in gallons per animal per year, assuming nothing is removed ahead of the lagoon: about 33 gallons per head of feeder-to-finish capacity and about 1,935 gallons per 1,400 lb milk cow (NC State AG-604, 2000). The same guide says sludge typically builds up over 10 to 15 years before it becomes a real problem.

Dairy lagoons fill faster for a reason you already know. University of Missouri Extension points to bedding and undigested fiber as the drivers of excess buildup, which is why dairies are especially prone to it. A properly sized hog lagoon usually avoids heavy buildup because swine manure is fine and breaks down easily, but an undersized or expanded one still fills (Missouri WQ324).

As the sludge rises, the liquid treatment zone above it shrinks. NC State notes that when treatment volume drops, both effluent nitrogen and odor go up. You lose storage, and the lagoon stops doing its job at the same time.

How do you measure sludge depth?

Do this before you call a contractor, and before you believe anyone's estimate, including ours.

A sludge survey has two readings at each point: the depth to the top of the sludge and the depth to the lagoon bottom. The difference is the sludge thickness. NC State recommends 6 to 24 measuring points per acre, with at least 8 for any lagoon under about 1.33 acres, laid out on a grid away from the sloped banks (NC State AG-639). On a 12-foot lagoon with 3:1 side slopes, the bank runs 36 feet out into the water, so stay well inside it.

The tools are not expensive:

Costs and accuracy as stated in NC State AG-639.

NC State specifically advises against judging depth by feel. Work from a boat with a flotation vest, and keep a second person on the bank with a throwable device and a phone.

For converting depth readings into gallons and into what that lost capacity is costing you, see how much lagoon storage capacity you are losing to solids.

When does sludge actually have to come out?

There is a regulatory answer and a practical one.

The regulatory answer comes from the NRCS Waste Treatment Lagoon standard (Conservation Practice Standard 359). It requires lagoons to be designed with room for residual solids, requires records of sludge accumulation and removal, and requires removal when sludge reaches the maximum residual solids storage level (USDA NRCS CPS 359, October 2017). NC State summarizes the trigger as sludge taking up more than 50% of the treatment volume, with an annual sludge survey once a lagoon has run five years. Your state permit may set its own timeline, so check it.

The practical answer is that sludge becomes a problem before it becomes a violation. University of Arkansas Extension notes that sludge left in place takes up liquid storage, which forces more frequent pumping and raises the risk of overflows and freeboard violations (Arkansas FSA1041). If you are pumping more often every year with the same herd, the lagoon is telling you something.

What are your removal options?

Agitate and pump

This is the most common routine method and usually the most economical. A custom applicator brings high-powered agitators, stirs the sludge into the liquid, and pumps it to tankers or a dragline.

The limits are real. Arkansas Extension says agitator range is commonly quoted at 50 feet or less, so a large lagoon can be left with an island of solids in the middle. Solids often resettle within about 30 minutes, which is why agitation should start hours before pumping and keep running during it. Pump agitators for earthen storage typically need a tractor of 90 horsepower or more.

A University of Missouri case study shows what a big job looks like. A dairy lagoon of about 7.5 million gallons that had not been cleaned in roughly 20 years was agitated and pumped over about three weeks in 2020, moving about 8 million gallons. Even after that, the report estimates more than a third of the original solids were still in the lagoon, and the main cost drivers were gallons pumped and hauling distance (Missouri G3401, 2022).

Draw down and excavate

For severe buildup with little water left, a trackhoe or dragline can dig the sludge out. Arkansas Extension recommends shielding or replacing the bucket teeth so the operator scrapes the bottom instead of digging into the liner. The material is drier, so it hauls with a box spreader instead of a tanker, but it takes more labor.

Dredge into dewatering bags

A floating dredge can pump thick sludge from a very large lagoon without agitating the whole thing. The sludge is often mixed with polymer and pumped into geotextile bags that let water drain back to the lagoon (NC State AG-881). It sounds like it solves the hauling problem. It only partly does. NC State notes that bagged sludge is still roughly 80% water, so it is still heavy to haul (NC State, 2021). Check your state rules before you stockpile dredged solids. Arkansas, for example, requires written approval from its environmental agency for that.

What does sludge do to your land base?

This is the part people underestimate. Sludge is not just thick manure. It is concentrated phosphorus, and for swine it carries zinc and copper too.

NC State's average values for active lagoon sludge put swine sludge at about 49 lb of P2O5 per 1,000 gallons and dairy sludge at about 22 lb (NC State AG-604). Real samples swing widely. NC State's 2025 data on pumped swine sludge ranged from under 2 to over 84 lb of P2O5 per 1,000 gallons, which is why they say to sample rather than trust book values (NC State AG-881).

That phosphorus usually sets the application rate, not nitrogen. Nebraska Extension suggests planning sludge applications to cover three to five years of crop phosphorus needs and steering toward fields with low soil-test phosphorus (Nebraska G1371). Arkansas Extension notes that applying sludge at agronomic rates takes noticeably more land than your routine effluent does. If your nearby fields already test high, the haul gets long and the bill gets bigger.

Sample every loaded tanker if you can, because composition changes as the pumping goes on, and feed the results into your manure management plan.

What can go wrong during a cleanout?

The liner. In an earthen lagoon the compacted clay and the solids pressed into its surface form the seal. Purdue Extension advises keeping the agitator at least 3 feet from the liner, or paving that spot, and repairing any damaged area (Purdue ID-303). Missouri Extension says to keep the agitator 2 to 3 feet off the bottom so the jet does not scour the clay. NRCS requires the liner type to suit the planned removal method, which matters most if you have a synthetic liner.

Leaving too little behind. The bottom layer holds the bacteria that run the lagoon. NC State recommends leaving the last foot, and Nebraska says the last two feet. After a full cleanout, NRCS calls for precharging with fresh water to the minimum treatment volume before loading resumes.

Odor. Inject or incorporate right away, spread on warm, sunny, breezy days, avoid still or humid evenings, and tell the neighbors before you start.

Gas. Agitation releases hydrogen sulfide. University of Minnesota Extension notes that at 100 to 150 ppm it deadens your sense of smell and is immediately dangerous to life (UMN Extension). Penn State advises a buffer of at least 20 feet during agitation, keeping children and bystanders away, and considering a portable gas monitor (Penn State, 2023). More on this in manure gas dangers.

What does it cost?

The published numbers are old, so treat them as a sense of scale, not a quote. NC State reported custom applicators charging 1.5 to 5 cents per gallon in North Carolina in 2000, depending on access, distance, and whether the sludge was injected. A University of Missouri dairy lagoon dredging job drew bids of $72,000 to $130,000 in 1993 dollars. Today's price on your operation comes down to the same drivers Missouri found: gallons, haul distance, and how many acres can take the phosphorus.

Can you slow sludge buildup?

Yes, mostly by keeping solids out of the lagoon in the first place. Settling basins and mechanical separators ahead of the lagoon reduce buildup. Research presented through the Extension swine community found that sludge accumulation drops by about the same percentage as the solids removed before the lagoon, so removing 30% of the solids cuts sludge buildup by about 30% (Extension Foundation). In the Missouri dairy case, the lagoon filled after the farm's separator fell into disrepair.

You should also know where the research stands on additives. NC State's sludge management guide reports that its evaluations of lagoon additives, starting in 1997, were unable to demonstrate significant reductions in sludge volume (NC State AG-604, 2000). Nebraska Extension notes that producer experience suggests some commercial products are effective at suspending settled solids so they pump out more easily, which is a different job from reducing sludge (Nebraska G1371). Arkansas Extension says additives are at best one part of a complete solids and odor management system, not a replacement for the other tools. We take all of that seriously. A biological program is not a substitute for a separator, a sludge survey, or a cleanout when the lagoon needs one.

Where PitCharger fits is the fiber. Dairy and cattle lagoons fill with bedding and undigested feed fiber, the material that floats into crust and settles into a bottom layer that agitators struggle to lift. Manure Digester is used for ongoing monthly treatment to help that material break down instead of building up, and LTC-E is built for high-fiber manure and crusting. A livestock producer in Mankato, Minnesota reported recovering approximately 200,000 gallons of usable lagoon capacity after treatment. Most producers should notice a difference within the first month, with continued improvement through the second and third months, and significant results are normally measured after about six months.

The honest way to judge it is the same way you judge any additive: survey the sludge before you start, survey it again after six months, and compare. If you want help setting that up, get a treatment plan or work your numbers in the calculator. For background on how lagoons work in the first place, see how manure lagoons work.

Frequently asked questions

How often should I survey lagoon sludge?

NRCS calls for an annual sludge survey once a lagoon has operated five years, according to NC State Extension's summary of the standard. Surveying the same grid points each year lets you see the trend instead of a single number.

Should I remove all the sludge?

No. The bottom layer holds the bacteria that keep the lagoon working. NC State recommends leaving about the last foot and Nebraska Extension says about two feet. If the lagoon is fully cleaned, NRCS calls for refilling with fresh water to the minimum treatment volume before you start loading manure again.

Is dredging better than agitating and pumping?

It depends on size and how bad the buildup is. Agitation and pumping is usually the most economical for routine cleanouts. Dredging with dewatering bags suits very large lagoons, but the bagged material is still about 80% water, and stockpiling dredged solids can need state approval.

Why does sludge need so much more land than regular effluent?

Because it concentrates phosphorus, and in swine lagoons zinc and copper too. Phosphorus usually limits the application rate, so the same gallons need more acres. Sample it and put the results in your nutrient management plan.

Can a lagoon additive replace a cleanout?

No. NC State's evaluations of lagoon additives, reported in 2000, were unable to demonstrate significant reductions in sludge volume, and no responsible supplier should tell you to skip a needed cleanout. A biological program targets the fiber that builds crust and bottom solids. Measure before and after so you can see what it is doing on your own lagoon.