Pesticides are any chemical substances that
are used to protect agricultural crops and livestock from pests for the promotion
of agricultural productivity and protecting public health. The pollution of soil, air and water and threat to human and animal
health are the major drawbacks in the application of pesticides. Treatment of pesticide polluted wastewater is, therefore, very
important. Biological treatment of pesticides offers the most economical option
if we compare it to other treatment methods. The
aim of the study was to recommend a safe and effective farm biological
treatment method that achieves a low level of pesticide contaminated wastewater.
In this study, the degradation of the fungicide captan was investigated
under both batch and continuous
operation modes with 15 days retention time. The results indicated that the initial cell number (30.1 ´ 106 cells/mL) in the soil-water mixture first decreased
with time for the first 24 hours and reached values of 15.6 ´ 106 and 11.1 ´ 106
cells/mL for the batch and continuous modes of operation, respectively. This
was the result of the inhibitory effect of captan on some of the microbial
species in the soil that are not tolerant to the pesticide at initial
concentration of 144 mg/L. Then, the microbial population started growing,
reaching its maximum after 5 and 12 days in the batch and continuous
bioreactors, respectively. The lag period and the specific growth rate for the
batch bioreactor were 22 h and 0.096 h-1, respectively. A captan
degradation efficiency of 89.6% was achieved after 10 days in the continuous
bioreactor compared to a degradation efficiency of 100% after 5 days in the
batch bioreactor. This study showed that the effluent from the continuous
bioreactor had a captan concentration of 12 mg/L which is not acceptable for
livestock drinking water. A half life of
52 h was observed in the batch bioreactor.
Keywords: Pesticide Captan, Bio-degradation, Bio-reactor, Soil
Microorganisms, Specific Growth Rate, Inhibition, Half Life.
Pesticides are primarily used for controlling undesirable organisms
(bacteria, fungi, mites, nematodes, insects, rodents in addition to undesirable
plants) that compete with humans for food and fibre or result in injury to humans
and agricultural products. Pesticides are classified according to the pest they
control into a number of classes (bactericides, fungicides, acaricides, nematicides,
insecticides, rodenticides and herbicides). The total amount of pesticides used
worldwide in 2007 has been estimated to be 2.37 billion kg this resulted in a worldwide
expenditures of 39.4 billion dollars. Pesticide expenditures represents about 13-22%
of the total production cost per hectare (Horowitz and Lichtenberg, 1993).
Osteen and Livingstion (2006) reported that for every dollar spent on pesticide
farmers get additional revenue of $4-33 as a result of increased yield,
depending on crop rotation and year of production.
Pesticides play an important role in increasing livestock and crop
production and permitting economical production of wide range of vegetable,
fruit, cereal, forage, fibre and oil crops which now constitute an important part
of any successful agricultural industry in many countries. Pesticides decrease
crop losses and increase revenue to farmers as a result of additional
marketable yield achieved because of the use of pesticides and consequently
lower the production cost per unit output (Horowitz and Lichtenberg, 1993).
Other benefits of pesticide uses include: (a) decreasing the uncertainty of losses
of crops from pests, (b) increasing profit to the suppliers of farm input
(machinery, fertilizer, chemicals and seed companies) as a result of increased
sale, (c) benefiting consumers because of decreased prices of raw foods or
improved quality of food products and (d) benefiting the whole society (farm
suppliers, farmers, food processors and consumers) from increased employment
and increased exports of food products (Oerke and Dehne, 2004; Cooper and
However, pesticides are toxic chemicals and if not properly managed
can have adverse effect on people, pets, livestock, wildlife and desirable
plants in addition to the pests they are intended to destroy (Centner, 1998;
Wilson and Tisdell, 2001). Pesticide residues remain in the containers and
application equipment after pesticides are used in the areas of interest. These
residues are removed by washing with water which results in the formation of a contaminated
wastewater, which represents a disposal problem for many farmers (Kearney et
al., 1988). Currently, the disposal of pesticide contaminated wastewater is
carried out by several methods including (a) land cultivation, (b) dumping on
land, in ditches, in lagoons and in soil pits, and sometimes in sewers and
streams near the rinsing operation, (c) use of evaporation ponds and (d) land
filling (Al hattab and Ghaly, 2012). These methods of disposal are considered
unsafe since the surface run off will ultimately reach surface water and the
infiltration of the wastewater into the local soil will eventually end up in ground-water.
The environmental impact of unsafe disposal of pesticides can be significant according
to the type and concentration of pesticides in the wastewater.
Therefore, pesticide contaminated wastewater must be treated properly.
There are number of treatment methods that are currently in use for pesticide
contaminated wastewater: (a) incineration, (b) chemical treatment methods as O3/Uv
oxidation, Fenton oxidation and hydrolysis (c) physical treatment methods that
uses absorbents and (d) biological treatment methods (phy-toremediation, composting
and bio-augmentation). These treatment methods either require land or are cost
ineffective and suffer from inaccurate effectiveness (Winterlin et al., 1989;
Al hattab and Ghaly, 2012).
The aim of this study was to develop an effective and safe biological
treatment for attaining low level agricultural pesticide wastewater in-situ and
to determine the mode of operation (batch vs. continuous).
The pesticide treatment system (Figure 1) consisted of a feeding
tank containing the pesticide contaminated wastewater, batch bioreactor,
continuous bioreactor and 2 overflow effluent tanks.
Figure 1. Experimental-apparatus.