Table of Contents
SECTION III: THE EFFECT OF BIOSOLIDS ON WATER QUALITY
3.2 Nutrient Loading
3.3 Organic Compounds and Trace Metals
Biosolids are managed by regulation to prevent any degradation of
water quality. This is because biosolids have the potential to
affect water quality through the leaching of at least two general
kinds of contaminants: essential plant nutrients and trace metals.
Ironically, these are the same components that were removed from
waste water to clean up discharges. The plant nutrients nitrogen and
phosphorous, when added to water, can cause algae blooms that may
lead to a degradation of quality. Excess nitrogen in drinking water
is a potential health hazard and the US EPA has established maximum
concentration limits for consumption of nitrate and nitrite. Trace
metals in water are of concern only when they are mobile and occur
in amounts above safe concentrations.
The US EPA and Maine DEP have definitive standards for selected
compounds in drinking water, but the ambient standards for
groundwater are less clear. The ambient standards are more
subjective, depending upon uses. The standard for groundwater of
highest quality (GW-A) is specified as being potable and suitable
for public water supplies. This implies that water quality must meet
state and federal quality guidelines established in the Safe
Drinking Water Act. The secondary standard (GW-B) states only that
the water be suitable for other uses. Clearly, the groundwater below
agricultural fields would not be used solely for public water
supplies. The lack of a numerical water quality standard for GW-B
generates confusion about the potential effects of biosolids on
water quality. The default comparison may be with the drinking water
standards, an inappropriate comparison. The impact analysis is made
more difficult by the presence of other agricultural chemicals
associated with manures and chemical fertilizers. A better method to
index impact is to compare changes in nutrients and metals in water
before and after land applications.
The goal of all land application programs is to add nutrients equal
to crop requirements and to prevent over-applications that could
lead to a potential loss of excess nutrients (USEPA, 2002). Sewage
sludges in Maine contain, on average, more than 4 per cent nitrogen.
The average nitrate plus nitrite concentration is less than 0.2 per
cent and average ammonium is less than 0.6 per cent. Nitrogen as
nitrate or ammonia is very water soluble and it is readily available
to plants. Movement of ammonia is slower than for nitrate or nitrite
because it can be adsorbed onto clay particles in soil. Being
soluble, these forms of nitrogen can be removed to varying degrees
in surface flow, or by transport down into ground water. Nitrate in
ground water is a ubiquitous problem in agricultural areas (Kellog
et al. 2000; Nolan, 2001). The Maine regulations are built on the
assumption that most of the nitrogen in the Maine sewage sludges is
bound in organic matter (~80%) and it is not immediately plant
available. The rate at which the organic matter decomposes
(mineralizes) and releases nitrogen is important for reducing water
quality impacts compared to inorganic fertilizers. A slow release of
nutrients provides tangible benefits to crops that need nitrogen in
steady doses. In general, nutrient release from biosolids is slower
than for chemical fertilizers or green manures.
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3.2 Nutrient Loading
Biosolids have been land applied successfully for many years because
they have fertilizer value (Hall and Williams, 1984). Studies of
nitrogen loss from field applications report that only 20 per cent
of nitrate is lost after several rain events (McLeod and Hegg,
1984). It appears that most biosolids have a small amount of soluble
nitrate that is released at initial application and then the slow
decay of organic matter releases nitrogen in quantities that is
quickly scavenged by crops (Chaney, 1990; Gilmour et al., 2000). At
a monitored biosolids spreading site in Colorado, nitrate in
groundwater was found to have no net change; modest increases or
decreases in concentrations over time balanced out (Stevens et al.,
2003). In New Hampshire, Estes and Zhao (1996) determined that
biosolids applied to cropland had a minimal effect on groundwater
quality because of the slow nitrogen release. McDowell and Chestnut
(2002) studied nitrogen loading at a topsoil manufacturing site
where biosolids were used. The only effect on groundwater quality
was detected near biosolids stockpiling locations. In Pennsylvania,
Shober et al. (2002) found that long-term biosolids application to
cropland increased soil nitrogen, implying that loss of nitrogen by
leaching was slight.
Higher amounts of plant-available nitrogen in soil are beneficial to
increase fertility for crops. Biosolids release nitrogen more slowly
than chemical fertilizers, so nutrient flushing is less of a
significant concern (Pierzynski, 1994). An extreme case of biosolids
use at high-application rates, at 1.5 to 5 times the agronomic
rates, at a gravel pit reclamation site, caused a quick flush of
nitrate at 50 mg/L into ground water followed by a lower but steady
input of 2 mg/L (Daniels et al., 2002). Gravel pits have very porous
soils and water movement can be relatively fast. In agronomic
applications of biosolids, excessive nitrogen addition is neither
allowed nor good farming practice.
If nitrogen from biosolids is not to affect groundwater quality, the
agronomic demand must be matched to the plant-available nitrogen (Kellog
et al., 2000; USEPA, 2003). Unlike manure or chemical fertilizers,
biosolids can only be applied in accordance with a written nutrient
management plan. There has been a considerable effort expended to
estimate the appropriate loading rate for initial nitrogen
utilization and subsequent releases of nitrogen during
mineralization. The amount of nitrogen available to plants will be
specific to each type of biosolids and the mineralization rates will
be controlled by site specific conditions (Gilmour and Skinner,
1999; Gilmour et al., 2000). The mineralization rate may have
half-lives ranging from hours to thousands of days (Overcash, 2004;
Overcash et al., 2005). Mineralization half life is the amount of
time needed for half of the starting material to be converted.
Estimating the amount of nitrogen that will become available to
plants is further complicated by the loss of some nitrogen as
ammonia gas (Pierzynski and Gehl, 2004). If nitrogen is to be
managed on a fine scale, the land application process will need to
have accurate loading calculations based upon soil chemistry, crops,
and biosolids. If application rates are too high, excess
available-nitrogen will be lost. For example, data from forested
sites indicated that nitrate loss to surface waters occurs after
biosolids are applied (increased over background by a factor of 2),
but ammonia export appeared to be constant (Grey and Henry, 1998). A
nitrogen mass balance was not reported in the study so nitrogen loss
may not be due solely to nitrogen applied in biosolids.
It is important for the nitrogen loading to match the character of a
particular biosolids. In Maine, as elsewhere, the problem of
nitrogen mineralization has been managed by the DEP using fixed
mineralization rates (Chapter 419, Appendix A). The mineralization
rate is based upon the type of sewage sludge, previous site
applications, and mode of use (topdressed or incorporated). The
Maine rules use a set amount of organic nitrogen that is mineralized
over several years as shown in Table IV. Reference guidelines for
Maine are based on Best Management Practices for Biosolids developed
by the University of New Hampshire Cooperative Extension (Boub et
al., 1995) and the US EPA (1983 and 1994). The formula involves
calculating the available nitrogen, and making allowances for
volatilization loss of ammonia if topdressed, and then correcting
for the nitrogen added by previous applications. This is a common
method for calculating loading rates.
TABLE IV. Mineralization of Organic Nitrogen From Sewage Sludge.
Values are per cent mineralized from initial application.
Years After Sludge Application
||Type of Sewage Sludge
|Table adapted from 06-096 CMR
Chapter 419, Appendix A
Nitrogen mineralization rates determine how much nitrogen becomes
soluble and plant available. Crop demand varies during the growing
season and the timing of application is important in order to match
the release of nitrogen with the uptake by plants. Rodriguez et al.
(2003) reported on biosolids providing adequate nitrogen for maize,
and mineralization provided 35 per cent of the nitrogen needed
during the following year. Biosolids mineralization rates were found
to be 20 to 50 per cent in a 36-week greenhouse study, consistent
with Maine’s rules (Adegbidi and Briggs, 2003). An analysis of
mineralization studies found that the biosolids application rate,
biosolids C:N ratio, and temperature were the master variables (Er
et al., 2004). These studies reflect the importance of
characterizing the applied biosolids and utilization site conditions
to manage nitrogen for crops.
One extreme example of nitrogen loss can occur when biosolids are
stockpiled prior to land spreading. In 2002 and 2003, research was
conducted in Maine to measure the loss of nitrogen from Class B
biosolids stockpiles (Peckenham, 2004). Stockpiles are a much more
concentrated source compared to spread biosolids and Class B
biosolids are expected to have a larger moisture content than Class
A. The stockpile experiment used plastic-lined cells to collect the
liquid running over or through stockpiles. Even though leachate may
contain an elevated concentration of nitrogen measured as total
Kjeldahl nitrogen (TKN), loadings were dependent upon leachate flow
rates. Loadings were calculated to be between 0.008 and 0.028
kilograms TKN/meter3/day (0.013 to 0.047 pounds TKN/yard3/day) in
the footprint of the stockpile.
The loading of TKN gradually increased over the first month of
stockpiling and reached a relative maximum at six to eight weeks.
Loadings decreased markedly after two months because leachate flow
decreased, even though concentrations of TKN in the leachate
increased. Although biosolids can show elevated concentrations of
nutrients and metals in leachate or run-off from a stockpile, they
also have a large capacity to retain moisture and reduce run-off
compared to soil (Glanville et al., 2004).
The loss of nitrogen from an unlined stockpile can have an impact on
soil and groundwater below the footprint. For example, assume a
field received a delivery of 100 cubic meters of biosolids for
land-spreading and this stockpile sat for 30 days. Based on the
nitrogen fluxes from the stockpile experiment, between 24 and 84
kilograms of nitrogen (as TKN) would be leached from the pile. Most
of the nitrogen is in the form of ammonia and this may become
converted to nitrate in the soil. This is enough nitrogen as
nitrate-N for 0.4 to 1.3 acres of hay, but being concentrated in one
small area could be lost to deeper soils or ground water. This
represents a concentration of nitrate to ground water beneath the
stockpile in the range of 240 to 840 mg/L. This concentration is
consistent with the data from New Hampshire where McDowell and
Chestnut (2002) found mean concentrations of nitrate in groundwater
wells below stockpile sites approaching 60 mg/L and soil solutions
had concentrations of 100 to 800 mg/L nitrate.
There is no absolute method to compare nitrogen species
concentrations in biosolids with concentrations in groundwater (Oertel
and Nicklow, 2003). This is because the process of nitrification
(conversion from ammonia to nitrate) is mediated by microbes and
rates depend upon soil conditions, as is denitrification (conversion
from nitrate to nitrogen gas). However, high nitrogen loadings, such
as under stockpiles, may increase nitrate-nitrogen in groundwater.
Similar nitrogen enrichment has been reported for manure lagoons (Gooddy
et al., 2002).
Biosolids contain phosphorous, but much of it is believed to be
contained in sparingly soluble forms (Coker and Carlton-Smith, 1986;
Elliot et al., 2002). According to Brandt et al. (2004) biosolids
contain water-extractable phosphorous, but in concentrations far
below chemical fertilizers or manures (USEPA, 2003). They report
phosphorous concentrations that range from 0.5 to 14 per cent of the
total mass. The limited mobility of phosphorous combined with its
varying content means that biosolids should be managed for
phosphorous on a case-by-case basis (Maguire et al. 2000). This
conclusion was substantiated by field studies over sandy soil
(worst-case scenario) that found no significant changes of
phosphorus concentrations in groundwater after sludge applications
(Shepherd and Withers, 2001). The evidence from agricultural regions
is that nutrients - from any source - are prone to be exported in
streams draining fields after excessive and multiple applications (Pyke
et al., 2003; USEPA, 2003; Richards et al., 2004).
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3.3 Organic Compounds and Trace Metals
There are two important points to be considered in relation to
organic compounds in biosolids- one has to do with the nature of the
compound and the other is how organic compounds interact with
metals. A few of the organic compounds detected in biosolids are
biologically active (act as hormones) or may be suspected of having
toxic effects. For instance, pharmaceuticals that enter into soil
and water have been shown to affect plant growth in laboratory
studies conducted at high concentrations (Jjemba, 2002). In general,
organic compounds that end up in sewage solids are sparingly soluble
(hydrophobic). This physical attribute acts to keep these compounds
from dissolving back into water. This means that the organic
compounds are not likely to end up moving into water. However,
organic matter can undergo chemical or biological processes that can
change the solubilities of daughter compounds.
The second aspect of organic compounds is that they may have the
ability to bind with metals. Stable organic compounds can serve as a
repository of metals that keeps them out of solution and otherwise
immobile. The long-term stability of organic compounds that bind
metals controls the release of many trace metals. The types of
wastewater treatment and biosolids formed, Class A or B, will
determine the characteristics of the organic constituents; factors
that may control metal mobility as organic matter decays (Stacey et
al., 2001). Antoniadis and Alloway (2002) determined that the
leachability of cadmium, nickel, and zinc were strongly affected by
organic matter derived from the parent biosolids to the point that
enhanced transport caused by soluble organic matter doubled the
distance these metals were leached through a soil column.
Several studies have investigated the fate of specific organic
compounds known to be from the land application of biosolids (Chaney
et al., 1996). Wang et al. (1995) tested a site that received 25
applications of sludge over 20 years. They determined that 90
percent of the chlorobenzene applied was gone and 10 percent was
detectable as a residual in the soil. Loss from the soil was
believed to be by volatilization and not leaching to ground water.
Loss from the soil was related to solubility as defined by the
octanol-water coefficient. Organic compounds that have high octanol-water
coefficients are less likely to enter into the ground water. Wilds
et al. (1991) found that polynuclear aromatic hydrocarbons (PAHs) in
biosolids persisted in soils for many years (half life 2 to 9
years). The key control on how much organic material could leach
into groundwater is the rate of mineralization (Jones and Evans,
2004). Some studies find specific compounds mineralize slowly:
Plasticizers (Madsen et al, 1999; Lindequist et al., 1999) and
Detergents (LaGuardia et al., 2001); while others found rapid rates:
Steroids (Mansell and Drewes, 2004; Snyder et al., 2004; Topp and
These studies suggest that overall, organic matter in biosolids is
relatively long-lasting. In addition, organic compounds may bind
with metals and keep them immobile. However, some fraction of the
organic matter in biosolids, along with some metals can be
transported in ground- and surface waters (Gooddy et al., 2002; Pyke
et al., 2003; Peckenham et al., 2004). Existing data are
insufficient to support strong conclusions about risks to
groundwater. Some connections between land uses and water quality
are likely to exist because of the effects of long-term agricultural
practices (chemicals, manures, and biosolids). Richards et al.
(2004) detected associations between organic matter and metals such
as sodium, copper, lead, and molybdenum in both soil percolates and
the baseflow of nearby streams.
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Biosolids contain water soluble compounds that could affect water
quality. Nutrients, organic carbon, and some metals can leach from
biosolids. It is important to stress that biosolids are mainly
derived from the least water-soluble components of the waste stream.
As biosolids age and decompose, all of the components are either
consumed by biota or transferred to the surrounding media (soil or
water or air). The use of good agricultural practices, including
soil erosion control measures, minimizes the impact of biosolids and
other nutrient sources on water quality (e.g. Mostaghimi et al.,
2001). The risks posed to surface and ground waters by spreading
biosolids are small when appropriate setbacks are utilized (Chaney
et al., 1996; USEPA, 2003). Uncovered stockpiles on bare ground will
leach small volumes of concentrated liquid that can affect
groundwater with leachate containing elevated concentrations of
nitrogen and trace metals.
The NRC (2002) study recommended performing multi-pathway risk
analyses that would include water. In addition, the study identified
the need for better monitoring and assessment of biosolids
utilization. The Biosolids Summit (Dixon and Field, 2004) also
stressed many of these same recommendations as well as the need for
new protocols to characterize the fate and transport of chemicals
from soil into water. Following a summary of the potential benefits
and deficits on water quality from using biosolids as a fertilizer.
- Required separation distances from surface water and biosolids
protect water quality.
- The thickness of soils and absorption onto soil particles protects
groundwater below fields approved for land application of Class B biosolids.
- Plant nutrients in biosolids are released slowly and are readily
consumed by plants.
- Metals contained in biosolids are retained by organic matter and
minerals in near-neutral soils.
- Nutrients from biosolids stockpiles can be leached to groundwater
or be too concentration for plant uptake.
- Soluble metals from biosolids may be transported to groundwater.
- Plants can incorporate potentially toxic metals from soil
- Long-term management of soil pH is needed to minimize metal loss.
Following is a relative assessment of how Maine’s rules protect soil
|Surface Water Quality
||Appropriate and adequate within
agronomic plan with proper use of setbacks. Restricted uses in
||Inappropriate applications on erodable land, or
excessive application when used with other unregulated nutrient
||Loading using appropriate
application rates are adequate when combined with good agronomic
nutrients via porous zones may lessen protection to groundwater.
Shallow groundwater table conditions (seasonal) may be vulnerable.
||Allow uncovered stockpiles on
solutions may transport nutrients and some metals rapidly.
Separation distance to groundwater beneath unlined stockpiles of
Class B biosolids may offer insufficient protection except for short
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