Abstract
Rapid urbanization has led to a dramatic increase in sewage generation
and sludge production in turn. There are very limited methods of
disposal and usage of sludge which are associated with drawbacks of
energy intensiveness and economic expenses. In the current study, sludge
from the Jagjeetpur sewage treatment plant (STP) has been collected,
composted, and characterized. A comparison of STP sludge compost and
compost made from farmyard manure showed the pH to be 6.67 which is
within the prescribed limit. The phosphate content was 1.013% which is
almost double the desired concentration. Few other parameters were weak
in comparison but drastic changes were seen upon analyzing the fortified
products. For the first product, Jaivik Poshak, the metal concentrations
were highly reduced after a dilution effect due to fortification. The
concentration of nine nutrients was higher in modified Jaivik Poshak.
For Jaivik Khad, the second product, almost all physicochemical
parameters were matched with the routine product and the Nitrogen and
Phosphorous content was higher than the prescribed limits making it more
suitable for fertilizer use. Phosphate was highly enriched in Jaivik
Prom due to the fortification. The Zinc, Boron, Manganese, and Sodium
were found to be higher in the amended product. Thus, it can be said
that sludge can be converted into valuable agricultural products with
minimal energy expenses and monetary benefits in addition. This can be a
viable option for the management of bulk quantities of sludge in an
eco-friendly manner to generate valuable returns for farmers and sustain
the environment too.
Keywords: Sludge management, Organic farming, Agriculture
Introduction
The semi-solid slurry generated from various industrial processes, from
wastewater treatment and on-site sanitation systems is known as sludge.
It may be generated as a settled suspension from drinking water
treatment, as sewage sludge from wastewater treatment, or as fecal
sludge from septic tanks. After treatment of wastewater in the treatment
plant, 99% of the water is recovered which is discharged as rejuvenated
water. The remaining 1% which comprises solids is the sludge.
Treatment of wastewater includes several stages. During preliminary
treatment which involves the removal of large particles like sticks,
stones, plastic, etc, the screened material is typically landfilled and
does not become a part of the sludge. Primary treatment comprises
gravity sedimentation and floatation during which half of the solids
that enter this stage are removed and become the primary sludge. It
comprises both organic and inorganic components. Secondary treatment is
a carefully controlled and biologically governed process where the
microbes degrade the organic components in the wastewater. The microbial
biomass generated as a result of this is organic in nature and becomes
the next component in the sludge. Tertiary treatment is done to reduce
the nitrogen and phosphorus component, suspended solids, and biological
oxygen demand. Precipitated components during the tertiary stage of
treatment add to the total sludge quantity. The final step includes
disinfection to kill the pathogenic microbes and discharge of the
rejuvenated water (Penn state extention, 2010). On average, dry sludge
contains 50-70% organic matter, 30-50% mineral content, 3.4-4%
Nitrogen, 0.5-2.5% Phosphorous, and several other nutrients depending
on the types of wastes treated and the stabilization processes involved
(Kacprzak et al., 2017).
Owing to the rapid urbanization and industrialization, the quantity of
sludge produced has become huge, and hence special attention is required
for its management. One aspect of
sludge management is its use in agriculture. Application of large and
uncontrolled amounts of untreated sludge results in the seepage of heavy
metals, phenolic compounds, polycyclic aromatic compounds into the soil,
groundwater, and surrounding water bodies which may bring about drastic
changes in soil fertility and damage to flora and fauna (Balkrishna et
al., 2022.; Houillon & Jolliet, 2005). It is important to pre-treat the
sludge, make it suitable to meet legal regulations, and then use it for
a particular purpose. However, pretreatment technologies are
complicated, expensive, and pose the question of the disposal of
concentrated pollutants separated from the sludge. Thermal incineration
is another alternative that significantly reduces the volume of sludge.
The ash thereby generated as a result of incineration needs other
methods for cyclization. The products of incineration may be used in
cementing industry or for vitrification (Cieślik, 2015). Other methods
exist for the recovery of valuable substances from the sludge. Pyrolysis
of sewage sludge under anaerobic conditions can be done to generate
adsorbents after the addition of various reactants. Other by-products
include pyrolysis oil which can be used as fuel. In this way, partial
recovery of sludge is possible (Cieślik, 2015; Samolada & Zabaniotou,
2014). Phosphorus recovery is another technology implemented by most
treatment plants.
The presence of heavy metals poses a problem for almost all sludge
management systems (Mailler, 2015). Recovery of such metals is done in
furnaces in oxidative or reducing conditions. Metals recovered in this
process are used in industries as catalysts or additions in special
products. This process is mainly done in developed countries owing to
the high-end instrumentation requirements, cost, and energy inputs
involved (Cieślik, 2015).
Underdeveloped nations primarily dispose of the sludge at the source
which thereby creates environmental problems and risks for humans and
aquatic life (Balkrishna et al., 2022). The wastewater treatment
processes concentrate heavy metals, organic pollutants, and pathogens.
Sludge disposal can therefore bring about the release of toxic
components into the environment which can thereby enter the food chain
(Chen et al., 2012; Wei & Liu, 2005; Pathak, 2009). Though landfilling
is an economical and low energy consuming process, it becomes a
significant source of CH4 and N2O which
are greenhouse gases (Peters & Rowley, 2009). Landfilling is also
incapable of utilizing any of the nutrients that are present in the
sludge. Landfill leachates containing P and heavy metals can also
significantly affect groundwater and surface waters. Land application of
sludge also has many benefits as it improves soil qualities and is
inexpensive. However, land application is also limited owing to its
heavy metal, micropollutant, and pathogenic microbe content (Wang et
al., 2008).
The current manuscript deals with how sludge may be converted into a
valuable agriculture input in an eco-friendly way. Three organic
fertilizers routinely made with farmyard manure compost have been
modified by using sewage treatment plant (STP) sludge compost. In our
study, expensive and energy-intensive procedures have been completely
avoided yet the product developed meets regulatory parameters. The
optimum fortification of sludge with suitable additives led to the
production of organic fertilizer with reduced heavy metal concentration
yet increased capacity for plant growth promotion. This procedure could
solve the problems of managing drastic amounts of sludge while creating
valuable agricultural inputs. It is proposed as a sustainable option for
converting waste into a valuable product for use in agriculture.
Materials and methods:
2.1 Site of sludge collection :
The sludge sample was collected from a sewage treatment plant at
Jagjeetpur in Haridwar, Uttarakhand, India. The location coordinates are
29.9015435, 78.1376691. This treatment plant has been commissioned since
November 2019 under the Namami Gange Program as a part of its
interventions to prevent the flow of untreated wastewater into the river
Ganga. The plant came under working conditions in June 2020. The total
treatment capacity of the plant is 68 million liters per day (MLD) and
uses sequential batch reactors for the treatment process. An average
sludge production reported for April 2021 was 399 MT (National Mission
for Clean Ganga, 2021). During our inspection to the site on 08.02.2022
and 15.02.2022, we found no tertiary treatment to be taking place in
this plant. As told by the head engineer of the site, the average sludge
production per day was approximately 19.68 MT. Also, the sludge
generated from the plant was deposited within the plant premises near
the Ganga River bank. It was found that the untreated sludge was
supplied to farmers on their demand for usage as agriculture input. The
head engineer of the site handed over the data regarding the inlet and
outlet wastewater parameters for January and till mid-February which
were collected by the auto analyzer as well as manually rechecked. With
regard to sewage, the data of solid particles (in %) in the outlets of
seven centrifuges for the period of January, 2022 was given. The extent
of dewatering of the sewage can be understood from that. The data for
MLSS in the six basins of the sequential batch reactor was also given
for January, 2022 which indicated the extent of suspended solids in the
sequential process and the extent of purification taking place after the
waste water crossed each basin.
Sample collection and characterization :
The sludge sample was collected in bulk in sealed packets and brought to
the laboratory. For the characterization of the sample, moisture
content, total organic carbon, pH, C/N ratio, Total Nitrogen content,
Total Phosphate content, Total Potassium content (as
K2O) and Total NPK were measured. The metal content was
analyzed using Atomic Absorption Spectroscopy (AAS). All the procedures
for the measurement of different parameters as well as metals were
followed from Fertilizer Control Order (FCO), 1985 (Government of India,
1985). A comparative assessment was done between the metal and macro and
micronutrient parameters of routinely prepared compost and compost from
the STP sludge to understand the nutrient status as well as heavy metals
status in both the compost.