WASTE MANAGEMENT ON CALIFORNIA DAIRY FARMS
A Comparison of Waste Management Systems
Melissa Rothstein, UC Davis School of Vet Med
July 31, 2015
I.
INTRODUCTION
State of the Industry
California dairies are home to approximately 1.78 million dairy cows on around 1,400 farms.1
California leads the country in milk production,1,2,3 producing over 20% of the country’s dairy
supply in 20124 and creating about $7.6 billion in sales in 2014.1 For the past few decades, the
industry has been trending towards fewer dairies in the state, but more cows per dairy and
higher levels of milk production per cow.5,6 In 2009, California dairies faced historic low milk
prices and high costs of production, resulting in a loss of 100 dairies across California.2
Challenges to Water Quality and Other Environmental Concerns
Dairies are a source of agricultural air and water pollution,5,7 and 47% of California’s water
footprint is associated with the meat and dairy industry, including the production of feed.8 The
industry might also contribute from eighteen percent9 to as much as fifty-one percent10 of
greenhouse gas emissions, and the San Joaquin Valley Air district has identified dairies as
significant sources of Volatile Organic Compound (VOC) emissions in the valley.11 Regulations
have been placed on the industry to mitigate the environmental impact of dairies, including the
General Order adopted by the Water Board in May 2007.12
Waste Management Systems
The degree of environmental impact from a dairy farm is largely dependent on the storage and
management of its manure.12 Manure management systems are complex and their efficacy is
influenced by many factors, including animal husbandry, feed, bedding, housing, climate, and
available land.6,13 This paper will focus on manure collection and processing. Methods of
manure collection in California include flushing, scraping, or some combination of these
systems, with flushing being the most common method in the Central Valley. The majority of
dairy farms (95.9%) have storage or treatment ponds, and many use some method of
separating solid and liquid waste.5
II.
STAKEHOLDERS
State Agencies
The storage and use of manure is a source of nutrients, pathogens, and salts that can adversely
affect surface and groundwater quality,12 while emitting greenhouse gases and other air
pollutants. While state and local agencies share common goals of protecting the environment,
public, economy, and industry, they have different areas of focus. For example, the Central
Valley Regional Water Quality Control Board (Central Valley Water Board) is highly concerned
with nitrates and salts, which can leach from lagoons or through soils after land application of
manure. The Air Resources Board (ARB), on the other hand, is concerned with methane and
other greenhouse gases (GHGs), while the San Joaquin Valley Air Pollution Control District (Air
District) focuses Volatile Organic Compound (VOC) emissions and air quality. CDFA is working
with CalRecycle to fulfill the requirements of the Healthy Soils Initiatives. A collaborative
solution brought about by each of these agencies working together and alongside the industry
could more broadly address environmental goals.
Animal Health and Welfare
Animal health and welfare are of importance to dairy farmers, their animals, and consumers,
because healthy animals produce milk more safely and efficiently. Two of the primary animal
health concerns in the dairy industry are the incidence of mastitis, or infection of the udders,
and hoof health or lameness issues.14,15,16 Both of these are best managed with prevention,
which primarily consists of keeping cows clean and dry.14,17,18
The gold standard for animal husbandry does not always coincide with the best environmental
practices or with what is most economically feasible, but should be a serious consideration in
management decisions. For example, sand is the ideal bedding for cow comfort and
health,13,19,20 but causes complications with most waste management systems.21 It is therefore
important to either invest in adequate sand separation technology or ensure a clean and dry
solid manure product for bedding.
Industry
In order for the diary industry to succeed and thrive, any solutions to be implemented must be
economically viable and practical. Due partially to unfavorable economic climate, coupled with
the sustained drought, the dairy industry has been changing and consolidating, and the CDFA is
concerned with keeping the industry viable. CDFA, along with farmers and industry
representatives, recommend voluntary and incentivized programs for reducing the impact of
the industry, as well as a variety of solutions that are flexible and appropriate to each individual
dairy.13,22
The Central Valley Dairy Representative Monitoring Program (CVDRMP) has identified a
framework for how to implement better nutrient management, which will include more
accurate measurements of irrigation water input, nutrient input, and nutrient removal;
improved record keeping of manure application amounts; and a means of measuring dynamic
nitrogen processes and organic N release in real time.23
The industry and its representatives have also expressed a desire for more consistent
relationship with the Central Valley Water Board, including clearer guidelines, better
interpersonal relationships, more positive feedback to farms that comply with or exceed
standards, and stronger enforcement actions for farms with violations. They recognize the need
for enforcement in addition to incentives and voluntary actions.22
Nonprofits
Many nonprofit and advocacy groups have concerns for the environment, wildlife, farm animal
welfare, and public health. These groups could potentially be used as a resource to conduct
research or build public support for environmental initiatives. For example, Sustainable
Conservation’s executive director, Ashley Boren, spoke at the July public meeting for the
Central Valley water board, and the group has completed research and put together an
extensive report on strategies to mitigate greenhouse gas emissions from dairy farms. 24
Given the environmental impact of dairy farms, there are some populations of people who are
disproportionately vulnerable to the effects of pollution, namely lower income communities
and immigrant workers who live or work in the San Joaquin Valley and close to farms. Groups
like the Center for Race, Poverty, and the Environment (CRPE) advocate for these groups. Some
waste management solutions have been proposed and marketed as beneficial to these
communities, though more accurately, the efforts are attempting to relieve part of the
pollution burden experienced by these communities. It is important to take these communities
into account when designing a solution and acknowledging the urgency of the problem.
III.
WASTE MANAGEMENT SYSTEMS
Flushing
Flush systems are those that clean manure out of the freestalls or barns by flooding water,
using large pipes, through the barns and into the lagoon. This water is usually recycled from the
lagoon, so it does not require more fresh water than what is already being used on the farm to
clean the milk parlor and cool the milk. Inspectors and farmers alike perceive this to be a
cleaner system that is better for the animals, as well as being easier to manage with less
maintenance and labor. Liquid manure is easier to apply than solid manure and can be applied
year round, rather than being restricted to times in between harvests.
Although some sources recommend flushing barns while cows are being milked and are out of
the barn,15 it is often done while cows are in the barn, which some believe helps to clean them.
Cows seem to prefer to defecate in the running water, allowing the manure to be washed
away.18 It is possible, however, that exposing the cows to bacteria-containing lagoon water
could increase the risk for mastitis15 or hoof infection, and this possibility warrants further
research.
Ammonia emissions can be reduced by diluting or acidifying manure slurry, or covering slurry
storage,25 and the use of more fresh water to flush barns can decrease N volatilization,3 so flush
systems could be beneficial. However, there is a concern about having large quantities of liquid
waste in lagoons that could potentially cause air and water contamination. Lagoons could
therefore be lined to prevent seepage and covered to reduce air emissions.
Scraping
Scrape systems use tractors or automatic blades to scrape manure out of barns. Scraping is
done either manually with a tractor, in which case cows need to be removed from the barn, or
with an automatic system that moves slowly and which cows can step over. Scrape systems are
mainly used in cold climates where flush water cannot be used because it would freeze in the
winter.
Scraping tends to reduce volatile solids (VS) and methane emissions from the liquid fraction of
manure waste,24 so ARB is interested in acquiring cap and trade funding to convert some flush
systems to scrape systems to lower these emissions. Scrape systems would also reduce the
volume and manure content of lagoons, and drier manure would be easier to transport,
compost, or digest. Switching from a flush system to a scrape system, however, would require
the addition of extensive infrastructure to manage the scraped manure properly, would require
large capital investment, and may not be favorable among farmers.24
One unintended consequence of scrape systems is that cows may slip due to the concrete being
scraped smooth (in the case of using a steel blade), which one farmer said resulted in a loss of
10% of his cows from slipping injuries alone. It is also possible for calves or downed cows to be
pushed into the manure pit in a system that is operating 24 hours a day without monitoring.26
In general, inspectors and farmers report that cows are dirtier on scrape systems due to poor
management of solid manure. 26 It is possible that using flush systems is why west region dairy
farms were found to have better cow hygiene and fewer severe hock injuries than east region
farms, where scrape systems are more common.27 Because of the mechanical components of a
scrape system, it can be harder to provide a back up in case of system failure than in a flush
system. Therefore, converting existing flush systems to scrape systems is not currently
supported by the data.
On the other hand, newer dairies being built could employ new scrape system technologies.
For example, vacuum scrape trucks might not be feasible on existing dairies because they
cannot necessarily fit the proper equipment or sustain the weight of the trucks on their
concrete. If implemented in new diaries, however, vacuum scrape systems would provide the
benefit of reduced water in the lagoons and would allow manure to be transported easily off
site. Vacuum scrape might also prove to be the cleanest option for cow health out of the
confined free stall systems. Vacuum scrape produces the lowest volume of manure, though one
study determined that a scrape system with flush flumes offers the best balance between
producing the lowest volume per cow and giving a consistent product to be utilized.13 This
option warrants further research.
Anaerobic Digesters
Anaerobic digesters can be used to reduce air emissions, odor, pathogens, and weed seeds, and
can produce renewable resources in the form of biogas or biofuels.21,28 They can be used with
either flush or scrape systems.21 The solid product provides a fluffy material to be used as
bedding or composted, while the liquid portion would contain fewer volatile solids (VS) and
emit less methane while being stored in a lagoon for land application, and nutrients might be
more plant available for use as a more predictable fertilizer.21 Current digester technologies use
combustion engines, however, which produce nitrogen oxide (NOx) emissions, another air
quality concern, so there is room for improvement in this technology. Digesters do not reduce
the nitrogen content of the liquid manure, which is then still stored in a lagoon. However,
digesters funded by the recent CDFA grants are required to install double-lined lagoons, which
provide water quality benefits.
Thus far, digesters are not always economically viable, because of conventional energy prices,
costs of interconnecting with utilities, lack of capital investment, and economies of scale.28
Viability can be improved through new technologies, such as the use of co-digestion, along with
governmental program support and capital investment, with the most economically feasible
system being a large-scale biomethane system with co-digestion.28 Revenue for the projects will
come from electricity, heat, biomethane, fertilizer, and compost produced, as well as
renewable energy credits and carbon reduction credits.28
Some caution should be exercised with this technology, because many of the digesters
previously funded by the federal government failed, leaving farmers with a negative impression
of their reliability. One reason for this may have been that digesters were not appropriate to
the existing waste management systems on the farms, so it will be important to tailor projects
to individual farms. Agency support should ensure that projects meet regulatory requirements
and receive adequate funding for both construction and maintenance.
Solid-Liquid Separation
Solid-liquid separation is an important aspect of waste management, because it could
potentially lower the solids content in lagoons and the quantity of liquid waste to manage, as
well as helping to produce a usable or saleable solid product. Although separators like settling
basins remove solids but not nutrients,7 liquid or semi-solid manure has a lower pH, which
slows ammonia volatilization, though this effect may be lessened through long-term lagoon
storage.29 Additionally, having fewer suspended solids in the lagoon water being used to flush
barns could have positive implications for animal health. While most farms in the Central Valley
already utilize some form of solids separation,5 this aspect of waste management warrants
further research and attention.
Some separators are mechanical while others utilize gravity, as in a settling basin or weeping
wall. Weeping walls have been shown to be more effective than mechanical separators or
settling basins,30,31 and are often preferable because they have fewer mechanical parts that
might require maintenance. A two-stage separation system allows for most effective
separation,31 and one case study showed that the largest amount of total Kjeldahl Nitrogen
(TKN) and Phosphorus were removed by two-stage separation process that included a
stationary inclined screen separator, gravity settling, and a pond additive.32 Pond additives have
also been identified as an area needing further research.5
Compost
Higher soil organic matter (SOM) content can increase plant productivity, water retention, soil
structure stability, aeration and water filtration, gas exchange and root development, nutrient
uptake, soil microbial communities, and carbon sequestration, and can reduce erosion and
leaching.33 Improved soils can be achieved through composting as well as strategies such as
cover cropping, crop rotation, and soil amendments. It will be important to consider potential
air emissions that might result from composting, and to employ impermeable surfaces to
prevent compost leachate from seeping. However, considering the many environmental cobenefits and relatively few risks, compost is a viable and cost-effective solution that can be
implemented now and with the cooperation of many agencies. CalRecycle and CDFA have
already hosted a joint interagency meeting to discuss this technology.
Pasture
Pasture is an option that is often seen as infeasible, but is highly desirable to many non-profits,
advocacy groups, and consumers. This has been successfully implemented in the northern
Sonoma area where the climate and topography are more suitable, but farms in the valley also
employ pasture grazing and receive a higher premium for their products when they receive
organic certification. This option warrants consideration.
I.
CONCLUSIONS/RECOMMENDATIONS
Good Management
Waste management systems are extremely complex and variable, and there is no silver bullet
solution that would be feasible on every farm. The solution will not be as simple as converting
from flush to scrape or building an anaerobic digester. Solutions should be tailored to individual
farms and their existing systems. Whatever the system, it needs to be managed very well to
mitigate environmental impacts.
Move Forward with Good Technologies
Composting has many co-benefits, and agencies are in a position to work collaboratively to
push compost forward in a manner that benefits all stakeholders. Solid-liquid separation is an
important part of the waste management system that warrants further research and
investment in technological upgrades. Anaerobic digesters should be constructed with support
from agencies so that they meet regulations and are sustainable. Finally, the industry is
researching better methods of measuring nutrient application, which need to be understood
and implemented as soon as possible.
Collaboration
This subject will require more research moving forward, which can be supported by outside
groups and institutions, including university scientists and faculty, non-profits, and farmers
themselves. Agencies must also work together and alongside industry and organizations to find
common solutions.
II.
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