Michelle A M Hopkins1, Adella M Kuster2, Jason R Vogel1,2* and Glenn O Brown1
1Department of Biosystems and Agricultural Engineering, Oklahoma State University, Stillwater, OK, USA
2Department of Civil Engineering and Environmental Science, University of Oklahoma, Norman, OK, USA
Submission: December 22, 2020;Published: January 19, 2021
*Corresponding author: Jason R Vogel, Department of Biosystems and Agricultural Engineering, Oklahoma State University, Stillwater, OK & Department of Civil Engineering and Environmental Science, University of Oklahoma, Norman, OK, USA
How to cite this article: Michelle A M H, Adella M K, Jason R V, Glenn O B. Pollutant Removal in Stormwater by Woodchips. Int J Environ Sci Nat Res. 2021; 26(5): 556200. DOI:10.19080/IJESNR.2021.26.556200
As urbanization continues, water pollution is of increasing concern for human health and the environment. Water contaminants common in urban stormwater include nutrients, metals, suspended solids, pesticides, and pathogens. The search for an inexpensive and readily available material that can effectively remove common stormwater contaminants is ongoing. Studies have shown that woodchips are a promising material that can remove many different contaminants, including these common contaminants and emerging contaminants of concern. The type of wood and shape of woodchips can impact the removal efficiencies of different contaminants due to different partitioning coefficients and capillary action. This review compiles studies on the ability of woodchips of different types to remove these common stormwater contaminants and emerging contaminants of concern. Overall, the literature demonstrated woodchips are an inexpensive and effective material that could be implemented for the removal of contaminants in urban stormwater.
Keywords: Water contaminants; Woodchips; Biological degradation; Pollutants; Aquatic organisms; Environment
In an ever-urbanizing society, water pollution is becoming more of a concern for human health and the health of the environment. There are many water contaminants associated with urbanization including nutrients, heavy metals, eroded sediment, hydrocarbons, and pathogens. As water pollution continues to increase, the search for inexpensive, readily available, and effective treatment techniques for remediating pollution in runoff is increasingly important. Woodchips have been investigated as an inexpensive treatment medium for many types of pollutants. Woodchips remove these pollutants by utilizing processes such as filtration, sorption, and biological degradation, and performance can be influenced by wood properties such as type of wood and shape. This review summarizes a variety of studies that demonstrate the use of woodchips for effectively removing a variety of pollutants. Utilizing this readily available material for pollutant removal from stormwater runoff could provide a low-cost, sustainable solution for water-quality improvement in stormwater runoff across the globe.
Common stormwater contaminants vary in chemical properties, resulting in different impacts on human health and
the environment and different removal processes. Sorption is one process woodchips utilize to remove pollutants. Woodchips are a porous material, so they contain small capillaries where water can flow by capillary action . As the water flows through the capillaries, pollutants sorb to the woodchips and are removed from the water. Woodchips also remove some pollutants through physical processes, such as filtration, where woodchips intercept the flow of water, allowing suspended contaminants to stick to the woodchips and in the pores of the woodchips, removing them from the water . Retention of pathogens in the woodchips can expedite deactivation of pathogens through natural decay, desiccation, or predation . Ion exchange can also occur when cations replace phenolic hydroxyl groups, found in the tannins in woodchips . Another process woodchips utilize to remove pollutants is biological degradation, which can occur in toxic or anoxic conditions. Most organic matter is degraded through oxidation by aerobic bacteria. The oxygen that is required for degradation of the organic material present in the water is represented by BOD or COD, so as organic matter is degraded, BOD and COD will decrease. Denitrification occurs in anoxic zones with low ventilation efficiency, such as the pores of woodchips or saturated zones . Denitrifying bacteria use woodchips as a carbon source and nitrates as a terminal electron acceptor, resulting in the conversion of nitrates to nitrogen gas .
Water quality indicators
Water quality indicators include biological oxygen demand
(BOD), chemical oxygen demand (COD), and suspended solids
and are indicative of poor water quality that may be caused by
pollutants such as excess nutrients, oils and grease, and/or
sediment. BOD and COD are indicative of organic material in the
water, and suspended solids can have other pollutants adsorbed
to them, so their removal is imperative. BOD and COD are removed
by biological degradation, and suspended solids are removed
through physical filtration by woodchips [6-10].
Excess nutrients in water can cause issues, such as
eutrophication. Many studies have found that woodchips
remove nutrients from water, including nitrate, sulphate,
ammonia, ammonium, nitrite, orthophorphorus, and particulate
phosphorus [2,3,5,6,9,11-17]. Nutrients vary in their chemical
properties, so their removal processes vary as well. Woodchips
act as the carbon source in the biological degradation of nitrate,
sulphate, ammonia, ammonium, and nitrite [5,13]. Particulate
phosphorus is phosphorus adsorbed to suspended sediment, so
it is removed by physical filtration along with suspended solids
. Orthophosphorus, is removed through sorption, and nitrates
can be removed by sorption as well as biological degradation .
Heavy metals can be toxic to humans and aquatic organisms,
and their presence can disrupt aquatic ecosystems. Mulch and
woodchips have proven effective for heavy metal removal, but
some metals, such as arsenic, have not been studied [4,13,18-21].
Metals are removed through sorption to the woodchips and cation
exchange with phenolic hydroxyl groups. The composition of the
wood can greatly affect the removal efficiency of the metals .
Pesticides often persist long term in the environment and
are detrimental to human health and the environment. Many
pesticides are organochlorides, which interact with the organic
material in the woodchips through sorption  or physical
filtration of sediment on which pesticides are sorbed . Several
studies have found woodchips to effectively remove pesticides
Total petroleum hydrocarbons
Total petroleum hydrocarbons describe a broad family of
chemical compounds associated with crude oil, including aliphatic
hydrocarbons, polycyclic aromatic hydrocarbons (PAHs), and
monocyclic aromatic hydrocarbons (MAHs) (ASTDR, 1999). They
have shown to be effectively removed by wood products in many
studies, by sorption to the woodchips as well as physical filtration
of sediments to which hydrocarbons are sorbed and biological
Other halocarbons have similar characteristics to pesticides
and have also been found to be removed by wood through
sorption. The other halocarbons studies include surfactants
, fluorene , 1,3-Dichlorobenzene, butylbenzylphthalate,
fluoranthene , trichloroethene , 1,2-Dichlorobenzene,
1,3,5-Trichlorobenzene, and chlorobenzene .
Pathogens present a risk to the health of humans and aquatic
organisms and are often indicative of fecal material in water. There
is a limited amount of research evaluating the ability of wood
to remove water-borne pathogens. Soupir et al.  evaluated
the removal of E. coli and Salmonella, and Rambags et al. 
evaluated the removal of E. coli and F‐specific RNA bacteriophage,
an indicator of viral pollution, by wood products, both finding
effective removal of these pathogens. Pathogens are removed by
sorption to woodchips or physical filtration if they are adsorbed
to sediments in the inflow, causing the deactivation of pathogens
by natural decay, desiccation, or predation .
Other emerging contaminants
Other pollutants that wood mulch can treat include explosives
such as Trinitrotoluene (TNT), Rapid Detonating Explosive (RDX),
and octogen (HMX) ; and emerging contaminants . The
chemical structures and properties and environmental impact
of these contaminants vary greatly, but they all have potential
of being removed by wood products. Less is known about the
interactions of these chemicals with woodchips, but many
emerging contaminants are likely removed by sorption (Table 1).
Woodchips are broadly effective for the removal of common
stormwater contaminants, but their effectiveness can be impacted
by both shape and type. The shape of the woodchips affects the
way water flows through the pore spaces in the woodchips, which
can affect sorption, ion exchange capacity, and even biological
degradation. Additionally, different types of wood have different
chemical compositions, which can lead to different sorption and
ion exchange abilities.
Shape of woodchips
Wood’s sorption and ion exchange capacity are impacted by
capillary flow, which is the movement of liquid by capillary action.
Washburn  defined capillary action for straight cylindrical
where l is the length the fluid traveled, γ is the surface tension,
D is the tube diameter, t is time, θ is the contact angle, η is the
dynamic viscosity, and K is referred to as the Washburn slope. The
Washburn equation, that assumes straight capillary tubes, can be
adapted for use in porous media that have tortuous connecting
pores. In fibrous materials, such as woodchips, the pore spaces are
irregular. This can cause variations in the effective pore diameter
and contact angle. Wålinder & Gardner  examine the factors
influencing effective pore radius and contact angle in spruce chips
with several different wetting fluids. They used fluids that have
low surface tensions, methanol and hexane, with an effective
contact angle of zero. From those experiments, the effective pore
diameter for the spruce chips was found .
Staples & Shaffer  present an equation that was catered to
capillary rise in porous media rather than using the Washburn
equation that was intended for straight cylindrical tubing. This
was done by testing the wetting front of saline in uniform glass
bead beds to find the simplistic flow front model,
where Dvis is the diameter at the throat that limits viscous drag,
ρ is the fluid density, g is the gravity constant, t is the time, and leq
is the equilibrium length, which is a function of surface tension,
contact angle, throat diameter, density, and gravity given by,
where Dcap is the diameter at the largest portion of the tube that
limits capillary pressure. More research is needed to determine
what shape and size of woodchips would have the highest removal
Type of wood
Trees can be categorized as either softwoods or hardwoods.
Softwoods are coniferous trees that produce their seeds in cones.
Examples of softwoods are cedar, redwoods, and pine. Hardwoods
are flowering trees that produce their seeds in fruit. Some
hardwoods are denser than others and are further separated
as soft hardwoods and hard hardwoods. Examples of soft
hardwoods include cottonwoods, balsa, and willows. Examples of
hard hardwoods include oak, hickory, and mahogany. Softwoods
generally have higher amounts of lignin than hardwoods. Lignin
contains polyhydric phenols and other functional groups on
its surface, making it important in the role of woodchips as a
sorbent for metals and hydrocarbons . Bailey et al. found
that sorption of metals, such as copper, chromium, zinc, nickel,
mercury, and lead on woodchips occurred primarily on the lignin
or tannin components (1999). MacKay & Gschwend  found
that two different softwoods, Douglas fir and Ponderosa pine,
had a high equilibrium sorption capacity for benzene, o-xylene,
and toluene. They also combined the work of Stamm & Millet
, Garbarini & Lion , Xing et al.  and Severtson &
Banerjee  to determine a relationship between the ligninwater
partition coefficient of the wood (Klignin) and octanol-water
partition coefficient of the chemical (Kow). The additional chemicals
include other petroleum hydrocarbons and chlorocarbons such as
phenol, trichloroethylene, dichlorophenol, and trichlorophenol.
The best fit regression for Klignin and Kow of the data that MacKay &
Gschwend  compiled is,
Lignin has been found to have a high sorption capacity for
hydrocarbons and metals, which makes woods with high lignin
content more efficient sorbents.
These studies have shown that woodchips are an effective
material for the removal of many different contaminants from
water. There are still some unanswered questions in the literature
regarding the pollutant removal capabilities of woodchips,
a) What is the effect of moisture content on the ability of
woodchips to remove contaminants?
b) What is the effect of external factors, such as humidity,
solar radiation, and wind speed on the ability of woodchips to
c) How well can woodchips remove other pollutants, such
as arsenic, that have not been previously investigated?
d) What shape and size of woodchips are most effective for
The literature has shown that woodchips can effectively
remove many different contaminants of concern that are
commonly found in urban runoff. It is a promising and inexpensive
material that could be widely implemented to reduce the transport
of contaminants through stormwater.