The management of liquid wastes is, in some ways, tied in
closely with drinking water quality. As I already discussed in that section,
agricultural and other liquid wastes can enter into surface and groundwater
sources for drinking water, as can pathogens from inadequate or improper
management of human and animal wastes. The subject of liquid waste management,
however, is much broader than just the safety of the water we drink and is
crucial to the health of entire ecosystems.
The first federal law in the US to address surface water
quality was the Water Pollution Control Act of 1948. This was expanded in 1956,
and in 1965, Congress passed the Water Quality Act, which provided for
assistance to states to meet specific water quality standards. That law was updated
in 1972 and 1987, and 1977 saw the passage of the Clean Water Act, which
provided for the development of technical programs and permitting processes to
limit and mitigate water pollution.
In part, these laws have provided federal funds for construction
and maintenance of municipal wastewater treatment plants. The Pollution
Prevention Act of 1990 further established a policy of preventing discharge of
pollutants at the source wherever possible, and the 2000 Wet Weather Water
Quality Act focused on preventing and minimizing the effects of sewer overflow
in storm events.
There are different paradigms for classifying liquid wastes.
One of these is the degradable-nondegradable classification scheme.
Degradable wastes include not only domestic sewage but also organic industrial
wastes. Nondegradable wastes are generally inorganic constituents of liquid
waste, such as salts and heavy metals. There are also persistent environmental
toxins, such as organochlorines (DDT, PCBs, etc.), which fit into neither category
neatly, as they degrade extremely slowly.
Because of the difficulties of easy categorization and the
potential for liquid wastes to contain both degradable and non-degradable
components, the more commonly used paradigm is that of point source vs. non-point
source wastes. The Environmental
Protection Agency defines point source wastes as follows:
“Any discernible, confined, and
discrete conveyance, including but not limited to, any pipe, ditch, channel,
tunnel, conduit, well, discrete fissure, container, rolling stock concentrated
animal feeding operation (CAFO), landfill leachate collection system, vessel or
other floating craft from which pollutants are or may be discharged. This term
does not include return flows from irrigated agriculture or agricultural
stormwater runoff.” (http://cfpub.epa.gov/npdes/glossary.cfm?program_id=0#P,
accessed 31 March 2012)
Non-point source wastes are, as the name suggests, any
wastes not fitting within the definition of “point source” wastes.
This paradigm is more useful because it provides a better
framework for designing mitigation and prevention strategies. Point source
wastes can often be addressed more effectively than non-point sources with
prevention strategies, such as improved processes to minimize primary waste
by-products. Additionally, treatment strategies can be more easily targeted to
the specific waste stream. Municipal
sewage, for instance, goes through a series of primary, secondary, and tertiary
treatment steps to remove suspended solids, aerate and oxidize remaining
organic matter, and additional treatments to render the effluent safe for
drinking or to release into surface waters. Industrial wastes, similarly, can
be put through a series of steps to remove particulates physically; to extract,
neutralize, or replace noxious substances through chemical means; and/or to
break down organic pollutants through biological processes.
During my time at NC State, one point source that was a
frequent topic of conversation within my department was waste lagoons from
feeder hog operations. North Carolina is one of the top pork-producing states
in the US, and the majority of the feeder operations, a type of CAFO, are
located in the Eastern half of the state, which is largely flat coastal plain
and more susceptible to flooding. Moeller cites overflow from the lagoons
during heavy spring rains in 2009, but I was living in the state in 1999 when a
large portion of that area was flooded by rains from Hurricane Floyd. Tabochow,
et al. (2001), documented the effects of the flooding of hog lagoons and of
municipal wastewater facilities on nitrogen and phosphorus levels in the Neuse
River Basin as a result of the hurricane.
Prior to Hurricane Floyd, much of our discussion was on the
potential consequences of trying to decommission the hog lagoons. They are
constructed with a clay lining, which remains more or less impermeable so long
as the lagoon is filled. If the lagoons were to be drained, however, the clay
liner would likely dry and crack, providing an entry point for accumulated
nitrogen and phosphorus to leach into the water table. The Neuse River drains
into the Pamlico Sound, which is the largest lagoon on the East coast and an
ecologically sensitive area.
Eutrophication of the waters in this area was and is a
significant environmental concern because of the potential damage to shellfish
and fishing stocks by algal blooms. JoAnn Burkholder is an aquatic researcher
at NC State who has done extensive research on the effects of eutrophication in
this area, and a 2004 article suggests that long-term leaching as described
above and non-point source runoff may actually be more damaging in this regard
than large flooding events such as were seen with Floyd, as the latter resulted
in massive flushing and dilution of nutrients and didn’t result in algal blooms
or any apparent long-term damage to commercial fisheries (Burkholder, et al.,
2004).
A greater concern from the standpoint of water quality degradation
is non-point source waste. Because this can come from numerous and
difficult-to-trace sources – lawn and agricultural fertilizers, urban runoff,
etc. – it is a much more insidious and difficult to control problem. In
addition to eutrophication, runoff from paved surfaces, such as parking lots
and highways, can carry petroleum distillates, salts, and heavy metals into
surface waters and pavement can exacerbate erosion by reducing surface porosity
and displacing organic matter and root systems that would otherwise help absorb
rainwater.
One strategy that has been of particular interest in
mitigation of non-point source, and some point source, wastes is that of
constructed wetlands, which are intended to provide a buffer zone between
runoff areas and flowing surface waters, as well as a potential sink for nitrogen,
phosphorus, metals, and other pollutants. With respect to point source wastes,
this strategy has been explored with some success for managing landfill
leachate systems (Nivala, et al., 2007; Białowiec, et al., 2012).
A 2011 study looking at constructed wetlands for removal of
pesticides from agricultural runoff, though, suggests that under certain
conditions, accumulation of the pollutants could occur which might be
problematic (Budd, et al., 2011). Further concerns about this approach are the
ultimate fate of pollutants like mercury, which can be converted from its inorganic
form to highly toxic methyl mercury by bacterial activity in wetlands, though
prevention at the source is likely the most effective strategy for minimizing
this risk.
Another consideration is the management of biosolids derived
from the wastewater treatment process. Sewage sludge is sometimes managed by
spreading it on agricultural fields, but there are concerns about the potential
for the contamination of food crops by pathogenic bacteria, as well as the
potential for heavy metals which are discharged into sewage systems to similarly
be redistributed into the food chain. Also, this process takes waste from a
point source and transfers it into a non-point source situation, which then
makes mitigation more challenging. Animal manure from agricultural activities
is also often handled in this manner, with much the same concerns.
Clearly this is a complex issue and this discussion does
little more than scratch the surface, but hopefully this gives at least a
glimpse into the topic.
REFERENCES
Moeller, D.W. Environmental Health, Fourth Edition. Harvard
University Press. 2011.
Tabochow, R.M., Peirce, J.J., & Essiger, C. “Hurricane-loaded
soil: effects on nitric oxide emissions from soil”. Journal of Environmental
Quality. 30:1904-1910. 2001.
Burkholder, J., et al. “Comparative impacts of two major
hurricane seasons on the Neuse River and western Pamlico Sound ecosystems”. Proceedings
of the National Academy of Sciences. 101(25): 9291-9296. doi:
10.1073/pnas.0306842101
Nivala, J., et al. “Treatment of landfill leachate using an
aerated, horizontal subsurface-flow constructed wetland”. Science of the
Total Environment. 380: 19-27. 2007. doi: 10.1016/j.scittotenv.2006.12.030
Białowiec, A., et al. “Nitrogen removal from landfill
leachate in constructed wetlands with reed and willow: redox potential in the
root zone”. Journal of Environmental
Management. 97: 22-27. 2012. doi: 10.1016/j.jenvman.2011.11.014
Budd, R., et al. “Removal mechanisms and fate of
insecticides in constructed wetlands”. Chemosphere. 83(11): 1581-1587.
2011. doi: 10.1016/j.chemosphere.2011.01.012
