NUI Galway – UL Alliance First Annual ENGINEERING AND - ARAN ...

Determination of optimal applications of municipal biosolids to soils to
optimize nutrient availability and minimize loss in surface runoff
J. Lucid a , O. Fenton b , and M.G. Healy a*
a Civil Engineering, National University of Ireland, Galway, Rep. Of Ireland.
b Teagasc, Johnstown Castle, Environmental Research Centre, Co. Wexford, Rep. Of Ireland.
Email: mark.healy@nuigalway.ie
Abstract
Biosolids are the by-product of urban wastewater
treatment and may be used as a fertiliser in agriculture.
While Ireland can be seen as one of the leading
countries in Europe in terms of landspreading of
biosolids, very little is known of the phosphorus (P)
availability of the biosolids to the soil or the potential
for nutrient release in surface runoff following rainfall.
The use of a novel agitator test in which an intact soil
sample is placed in a glass beaker, overlain with a
known volume of water and agitated to simulate
overland flow, can be used to determine the release of P
to surface runoff.
1. Introduction
In Ireland there are approximately 86,000 tonnes of
biosolids produced per year, of which over 70% is
presently disposed off via landspreading. Provided that
the biosolids are treated to the approved standards, they
can be landspread in agriculture, and offer an excellent
source of phosphorus (P), nitrogen (N), and metals
required for crop growth.
The Landfill Directive, 1999/31/EC (EC, 1999),
requires that, by 2014, the disposal of biodegradable
municipal waste via landfill is to be reduced to 35% of
the 1995 value. As a direct result, landspreading of
biosolids provides a sustainable and beneficial
alternative to landfilling.
Guidelines governing their use are based on research
carried out abroad and little information is based on
their interaction with Irish soils. In addition, the
guidelines do not consider the relationship between
biosolids application rates, nutrient availability, and
surface runoff of nutrients, suspended sediment (SS)
and metals.
2. Materials and Methods
The soil used in this study was collected from a local
farm in Co. Galway. Biosolids were collected from
three wastewater treatment plants in Ireland. They were:
lime stabilised biosolids, anaerobically digested (AD)
biosolids, and centrifuged biosolids. They were tested
for their nutrient and metal content.
The following treatments were carried out in
triplicate (n=3) in the agitator test: grassland only;
grassland receiving centrifuged, lime stabilised and
anaerobically-digested biosolids. All treatments were
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applied at the optimum application rate for the soil
under investigation.
The soil, 40 to 50 mm in depth, was first transferred
from aluminium cores to glass Pyrex cylinders. The
biosolids were then applied to the soil surface (t=0hr)
and left sit for a period of 24 hr to allow the treatments
to interact with the soil. After this 24 hr period, the
samples were then saturated by the gradual addition of
deionised water over a 24-hr period. This was
conducted until slight ponding of the water was seen on
the surface of the soil. After this 48-hr period, 500 ml of
deionised water was gently added to the beakers. An
agitator paddle was then lowered to mid-depth in the
overlying water and rotated at 20 rpm for 30 hrs as an
attempt to simulate overland flow.
Throughout the 30-hr period of the test, 2.5 ml water
samples was removed at mid-depth of the overlying
water at pre-determined time intervals and tested for
dissolved reactive P (DRP). A 15 ml sample was
removed from each beaker at the end of the test (at 30
hr) and stored until metal analysis was carried out.
3. Results
Initial testing has shown that the centrifuged
biosolids released, at their peak, 2.43 mg DRP L -1 ,
which equated to a mass of P released in the overlying
water of 148.464 mg m -2 of grassed surface area, while
the AD biosolids released 0.36 mg DRP L -1 (22.2 mg m -
2 ). The control results were far more conservative,
releasing 0.14 mg DRP L -1 (8.86 mg m -2 ).
4. Conclusions
For all types of biosolids analysed, P levels in the
overlying water are in excess of the maximum
allowable P concentration in surface water (20 – 30 µg
P L -1 ). Furthermore, owing to the centrifuged biosolids
releasing eight times as much P as AD biosolids,
biosolids may potentially need to be mixed with a
chemical amendment capable to adsorbing P (alum,
ferric chloride, etc.) before being landspread.
5. References
[1] Council Directive 1999/31/EC of 26 April 1999 on the
landfill of waste.