Lagoon Treatments

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Waste storage lagoons are a source of concentrated emissions. Various techniques have been suggested to reduce emission including, digesters, covers, aerators, and additives, but many of those suggestions are expensive and few work efficiently.

The Problem
Background
Experimental Methods
Results and Cost Anlaysis
The Solution

The Problem

After solid separation, 75% of the total nitrogen in collected manure goes into the lagoon with the liquid portion of the influent (19). At a typical lagoon pH of 7.0-7.6 over all seasons, 60% of the total nitrogen is expected to be volatilized as ammonia (19). A portion of the remaining nitrogen in solution is either lost as nitrous oxide and nitrogen gas following nitrification/denitrification, or retained in the lagoon as nitrate or other non-gaseous nitrogen compounds (23). Since most lagoon effluent is used as flush water for barns, the remaining nitrogen retained in the lagoon water is returned to the barns and usually volatilized as ammonia. Thus, with a recycling system, the nitrogen loss potential is near 100%. If the lagoon water is used for irrigation, this volatilization potential is less (about 50%) due to nitrogen application to fields (19) and uptake by plants.

Background

The rate of ammonia volatilization from lagoons will vary with temperature, nutrient load, pH, the presence of a cover or crust on the surface, and the aerobic/anaerobic status of the lagoon.

The most common type of lagoon found on dairy operations is a single or double anaerobic lagoon with no treatment. This is also the easiest and most cost effective lagoon management stategy. However, this type of ayatem may also have the greatest rate of ammonia emissions. The following treatments have different cost and managment imputs, but all show promis to reduce ammonia emissions from anaerobic lagoon systems.

pH Manipulation. If the pH of the lagoon is maintained above 8 (basic), ammonia volatilization increases and ammonia volatilization may be up to 70% of the total nitrogen entering the lagoon (19). At a pH below 6 (acidic), ammonia is bound in solution in its ionic ammonium form and little ammonia volatilization will occur (24). Achieving a low pH requires the addition of acidifying compounds such as alum, citric acid, and nitric acid to the lagoon. Positive results have been found in reducing ammonia emissions from small-scale waste confinement and laboratory studies, but large scale studies are limited due to cost and practicallity. In addition, low pH reduces the efficacy and bacterial activity of anaerobic lagoons and increases odor. Finally, the acidic lagoon water can have detrimental effects when applied to crops later.

Temperature. An increase in ambient temperature will increase the rate of ammonia volatilization from waste lagoons (23, 24, 25). Aneja et al. (24) found a strong correlation between ambient temperature and ammonia flux from the surface of swine lagoons. They reported that the greatest emission of ammonia was in the summer months, which was up to 60% of the total yearly flux. Safley and Westerman (26) found similar results from dairy waste lagoons, noting that the greatest ammonia emissions were in the warmer summer months. Likewise, the lowest ammonia volatilization is in the winter when microbial activity in the lagoon is dormant. This would suggest that lagoons should be kept at lower temperatures to reduce ammonia emissions, but his method is not practical nor feasable on large scale, in-ground lagoons.

Aeration. Most animal waste lagoons are anaerobic in nature, and therefore essentially all of the nitrogen entering the lagoon is lost as volatilized ammonia due to nitrification and denitrification processes (23). An alternative to anaerobic conditions that would help reduce ammonia volatilization from lagoons is aeration. Aerated lagoons are oxygen-rich and promote the process of oxidation, oxidizing ammonia to nitrate. Rarely, however, are livestock waste lagoons totally aerobic, as aeration is difficult to achieve in a livestock lagoon due to the high solids and protein content in the slurry (27), and costly energy input. Rumburg et al. (28) installed commercial aerators in a dairy lagoon and found no change in ammonia emissions stating that the aerators failed to introduce enough oxygen into the lagoon to degrade the ammonia. Due to the high oxygen demand of the nutrient rich solids in a lagoon, it is difficult to provide enough oxygen (1-2 mg/L) to achieve proper aeration in a waste lagoon (27, 28).

Facultative Bacteria. One of the best options to reduce ammonia volatilization from lagoons is to have a facultative or stratified lagoon, which has a top layer of aerobic activity to reduce ammonia and odor emission, and a bottom layer of anaerobic activity to promote microbial breakdown of solids and nutrients. This is achieved by mechanical circulation/aeration of the top layers of the lagoon, or can occur naturally in swine lagoons where solids are low, in secondary dairy lagoons, or overflow lagoons with low solids content and nutrient load (27). It was found that lagoons that were partially aerated or circulated tended to cultivate nitrifying bacterial populations that helped reduce ammonia in the lagoon water by oxidizing ammonia to nitrite and nitrate. In order for this process to take place, the lagoon must be kept at a pH between 7 and 8 to maintain bacterial populations and minimize ammonia volatilization.

Covers. The rate of volatilization from the surface of a lagoon relies on environmental factors such as ambient temperature, relative humidity, surface wind velocity, and precipitation. To control these factors, the addition of a cover to the lagoon can reduce uncontrollable variables and capture unwanted emissions. A cover can be a floating plastic cover, a synthetic or natural cover of peat, straw, or polystyrene, or a natural cover formed by the presence of dry matter in the lagoon. When working properly, synthetic covers can reduce nitrogen losses by 80-90% (19), but any cracks in the cover will greatly reduce this efficiency. Additioanlly, the installation of synthetic covers can be expensive and laborious. The formation of a natural crust on the top of lagoons can decrease ammonia emissions by to 50 to 60% (13, 41), and requires very little management. The natural crust development occurs as a result of fiberous solids in the lagoon being carried to the surface by methane or carbon dioxide gas bubbles generated by microbial degradation of the organic matter in the lagoon. Evaporation at the surface of the lagoon promotes the drying of the solids and formation of the crust. The formation of a natural crust will occur when the lagoon has a high solids content, the ambient air is dry, and there is little action or precipitation to break the crust, as in Colorado. However, it must be noted that natural crusts still need to be managed. Crusts left too long (longer than one year) without being broken up can reduce the capacity of the lagoon storage, may encourage rainwater runoff from the crust surface instead of transport into the lagoon, and may become difficult to break up when the lagoon needs to be pumped out.

Experimental Methods

Method. We will be evaluating various lagoon treatments that are already in operation on dairy operations in CO. Some of these treatments include, aeration, natural crusts, and improved solid seperation.

Results and Cost Analysis

Results. Coming Fall 2008....

Cost Analysis. Coming Fall 2008....

The Solution

Coming Fall 2008....