The way the urban biowaste is treated represents a challenge and a great opportunity all over the world. The organic fraction usually constitutes the dominant part of our waste. Statistics confirm that worldwide it accounts for 46%, up to 64% in developing countries, while it drops to 28% in higher-income ones.

The organic fraction of municipal solid waste (OFMSW) includes food and kitchen, leaves, grass and flower cutting and yard waste. Especially the first constitutes a significant part of this organic material.

Urban biowaste improperly treated produces environmental pollution, can pose a risk to human health and it increasingly requires more space for their disposal.

On the other hand, the recycling of biowaste represents a desirable alternative to landfill. If properly treated, urban biowaste can be used to produce biogas or compost, by transforming a potential garbage into a precious resource.

The European Commission recently adopted an ambitious circular economy package, which includes measures to guide Europe’s transition towards this economic model: one of the various objectives of the EU countries plans to recycle 65% of urban biowaste by 2030 and to dispose of a maximum of 10% in landfills.

To achieve these objectives, the technologies applied to the treatment of urban organic waste become a real strategic resource and allow to fully exploit the recycling potential of the urban biowaste generated by a large number of activities.

What are the treatment plants?

All treatment plants of urban waste tend to be organized in the same way: there is an area for the reception of waste, another for pre-treatment and mixing, a third area for the actual treatment (which can be different, according to the final product of the recycling process and the type of plants) and finally the storage area of the final product.

The plants mainly differ in the type of treatment which can be:

• mechanic;
• thermal;
• chemist;
• biological and biochemical;
• ultrasound;
• microwave;
• high voltage pulse discharge.

The mechanical process is particularly suitable for the production of biogas. It works by crumbling the pre-treated and mixed organic fraction, until it is reduced to extremely small particles. It increases the accessibility of the surface to microbes and improves the availability of nutrients, increasing the anaerobic process.

The heat treatment of urban biowaste modifies the structure of the insoluble part of the substrate and makes it easily biodegradable. It promotes the elimination of any pathogens, improves dehydration and reduces the thickness of the digestate.

The chemical process is used to break down the connections in the cell wall of plants by using strong acids, alkalis or oxidants. It is more suitable for substrates that are difficult to biodegrade and it is often practiced by using ozone.

Through the application of enzymes and the exploitation of already present microbes, the biological and biochemical treatment of biowaste promotes microbial growth on biomass and greatly improves substrate hydrolysis. This increases production both in the case of biogas or methane and in the case of compost.

The ultrasonographic treatment uses ultrasound to improve the hydrolysis and acidogenesis of the organic fraction.

Using microwaves in the treatment of biowaste is particularly useful for promoting the hydrolysis of non-biodegradable natural substances that resist anaerobic treatment.

According to some studies, the high voltage pulse discharge treatment is particularly effective in increasing the cumulative methane yield.

What are the stages of the OFMSW treatment?

The OFMSW treatment takes place in special plants in various stages that may partly differ depending on the type of product to be obtained from recycling, such as compost or biogas.

The stages that are common to the two production paths are:

• receipt of materials;
• pretreatment and mixing.

In the first stage, the OFMSW enters an area of the plant which is generally composed of a weighing machine for the incoming material and an unloading area. The latter can be buried to avoid the dispersion of unpleasant odors.

The preliminary treatment phase mainly performs two functions: 

• eliminating any foreign materials (e.g. plastics, sands, etc.), which could sediment and create clogging problems;
• preparing (by shredding, diluting or adding structuring matrices) a substrate with a dry matter content that is compatible with the final product and with the adopted technology. For example, in compost production, wood and cellulose waste, previously shredded, is added. 

In this case the OFMSW treatment continues with the stages of:

• bio-oxidation;

• maturation;

• refining and storage.

During the bioxidation stage, the pre-treated and mixed organic fraction is humidified, turned over and subjected to air insufflation, so to easily and quickly reach the stabilization of the mass (CO2 production) and increase the temperature of the material.

In the maturation stage (which has a minimum indicative duration of 45 days) the synthesis of humic molecules takes place, giving the compost the characteristics of a soil conditioner.

Refining and storage is the last stage. It requires that the material obtained is screened before being commercialized, in order to remove coarse or unwanted parts. At the end of this step, the compost is similar to undergrowth soil and practically odorless.

Instead, if the result of the recycling procedure is biogas, the OFMSW treatment will continue with the stages of:

• anaerobic digestion;

• energy production.

In turn, anaerobic digestion involves three steps:

• hydrolysis and acidification, which is the degradation of the more complex molecules (proteins, fats and carbohydrates), with the formation of amino acids, fatty acids and simple sugars, and subsequent transformation into volatile fatty acids, alcohols and ketones, which constitute the substrate for the subsequent phase;
• acetogenesis, or transformation of fatty acids and alcohols, formed in the previous phase, into acetic acid, formic acid, CO2 and H2;
• methanogenesis, or the formation of methane starting from acetic acid, or from molecular hydrogen, thanks to specific bacteria.

The biogas that has been produced is finally ignited in engines that drive power generators. In this way it is possible to recover both electricity and heat.

At what stage are our pumps used and why?

Let’s now analyze in detail at what stage our pumps are used and why.

The technologies used in the treatment of urban organic waste represent a crucial factor in being able to make the most of their recycling potential, reducing environmental impact, health risks and land consumption for disposal purposes.

In urban biowaste transformation plants, our pumps are particularly suitable for:

• stage of transferring the pre-treated and mixed organic fraction to the storage and maturation tanks;
• feeding of anaerobic digesters;
• recirculation of the digestate, both in a mesophilic and thermophilic environment.

All models of Perissinotto’s Pemo pumps can effectively operate in any stage of the municipal biowaste treatment process. Their strengths are undoubtedly the wear-resistant components, the particular geometric design of bodies and impellers (able to prevent clogging), and the flow rates that reach 1,500 cubic meters per hour.

Our experience in the field of industrial pumps also allows us to create unique products, which are customized according to the needs of those who work in the field of OFMSW treatment or in any other industrial sector.