Vermicomposting of sludge from a malt house

Highlights

• Earthworms were able to convert sludge from a malt house into a value-added product.

• The addition of moistened straw pellets at a minimum of 50% (vol.) was necessary.

• Vermicomposts were characterized by optimal agrochemical properties and maturity.

Abstract

Malting sludge is waste that could be used as a good soil conditioner after proper treatment. In the current study, the feasibility of vermicomposting malting sludge and its mixtures with straw pellets on the basis of physico-chemical and biological properties was verified. A vermicomposting system with continuous feeding of earthworms Eisenia andrei was used. The greatest number and biomass of earthworms was found in the variant with 25% malt house sludge + 75% straw pellets (on average of all layers: 320 earthworms/kg and 35 g/kg, respectively), followed by a variant with 50% malt house sludge + 50% straw pellets (on average of all layers: 47 earthworms/kg and 13 g/kg, respectively), indicating that a minimum of 50% (vol.) straw pellets is necessary for successful vermicomposting of malting sludge. Most earthworms lived in the youngest upper layer (42% and 52% of total number and earthworm biomass, respectively). On the contrary, the oldest bottom layers (final vermicomposts) after 180 days of vermicomposting were characterized by maturity, indicating lesser contents of microorganisms and enzyme activity. These vermicomposts had favorable agrochemical properties (pH = 7.8, EC = 1.2 mS cm⁻¹, C/N = 11, P_tot = 1.23%, K_tot = 2.55%, Mg_tot = 0.42%). The proportion of the available contents in the total contents were 10%, 59%, and 19% for P, K, and Mg, respectively.

Introduction

Due to the loss of soil organic matter in many countries, it is appropriate to improve the balance of organic matter in soil (FAO and ITPS, 2015). To achieve this goal, it is necessary to use all sources of organic matter, from which high-quality organic fertilizers are produced. One of the non-traditional valuable organic fertilizers can be a vermicompost based on malting sludge.

Malt is formed during the physical and biochemical changes, which occur in endosperm of barley grains (Ramesh and Rosell, 2017). It is one of the principal ingredients in the manufacturing of beer whose global production amounted to about 2 billion hectoliters in 2018 (Statista, 2019). The malting process produces a large amount of malting wastewater (Mielcarek et al., 2013). After treatment of the wastewater, the malting sludge remains. Direct application of malting sludge on the land is hardly feasible due to the gelatinous structure and very strong odor. The common disposal route for dewatered malting sludge similarly as in the case of brewery waste water sludge is a landfill (Kanagachandran and Jayaratne, 2006). Landfilling is the least preferable option in waste hierarchy. Along with land contamination and odor issues, landfill leachate consists of concentrated and recalcitrant substances that can seriously pollute the nearby groundwater aquifers and surfaces. Thermal waste-to-energy technologies (combustion, gasification, pyrolysis, torrefaction), and waste biorefinery techniques (e.g. anaerobic digestion (AD)) are better options for biogenic waste handling (Tsui and Wong, 2019). AD involves a collective microbial degradation process of biowaste in the absence of oxygen, and has recognized potential to contribute to high-volume biogas production. New ways to optimize anaerobic bioreactor performance are sought and tested (Tsui et al., 2018, Tsui et al., 2020). Due to increasing environmental concerns, it is desirable to utilize the digestate from AD or untreated sewage sludge in an environmentally friendly manner. Composting is a possible solution for handling of this waste. The loss of NH₃ during the thermophilic stage of the process is one of the major drawbacks of the composting process (Koyama et al., 2018). An alternative to composting is the vermicomposting process (Lee et al., 2018). The vermicomposting process involves the biooxidation and stabilization of organic materials by the joint action of earthworms and microorganisms. Although it is the microorganisms that biochemically degrade the organic matter, earthworms are the crucial drivers of the process as they promote aeration conditions, fragment the substrate, and thereby drastically increase the microbial activity (Dominguez and Edwards, 2011). Compared to the feedstock and conventional compost, vermicompost is characterized by increased and more soluble levels of major nutrients and organic matter with improved quality. In addition, this product contains plant growth hormones, enzymes, and substances that are able to protect plants against pests and diseases (Sinha et al., 2010).

Vermicompost can be applied as a soil amendment to improve soil fertility and structure by increasing the soil organic matter, cation exchange capacity, nutrient content, and water-holding capacity (Arancon et al., 2006, Xu and Mou, 2016). Vermicompost reduces soil erosion, water runoff, soil detachment, and NH₄⁺ and NO₃⁻ transfer to water (Doan et al., 2015). Many studies have indicated that vermicompost is preferable to compost to improve soil quality and economics of organic farming (Fornes et al., 2012, Tognetti et al., 2005, Lim et al., 2015).

At this time, there do not appear to be any scientific studies on the vermicomposting of malting sludge. Keeping the above facts in mind, the present study was implemented to investigate the feasibility of vermicomposting malting sludge and its mixtures with straw pellets. For this purpose, a set of methods was employed to describe processes during vermicomposting in terms of: 1) earthworm survival and development; 2) physico-chemical properties such as dry matter, pH, electrical conductivity (EC), total and available content of nutrients; and 3) biological parameters determining the representation of the main groups of microorganisms and the activity of hydrolytic enzymes.