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.