SOLAR GREENHOUSES FOR DEPURATION SLUDGE DRYING – Copparo (FE), November 2021
AGquadro Srl for the municipality of Copparo
1. PREMISES
One of the main problems in the management of purification plants concerns the disposal of sludge linked to various causes listed below:
• Regulatory uncertainty about the final fate of the sludge based on its characterization.
• Lack, at a national level, of an adequate number of recovery sites or landfills capable of satisfying sufficiently the need for the final delivery of dehydrated sludge from purification plants.
• Important seasonal variability of the acceptance of sludge by composting / recovery sites.
A possible approach for containing disposal costs is to increase the percentage of dry matter to be removed by reducing the water content in the mud.
An investment consists in reducing the volumes of sludge through the construction of solar drying greenhouses. This approach to mud is of great interest in two respects. Firstly, because the drying of sewage sludge is a proven technique to date that allows to obtain granular sludge starting from dehydrated sludge with a weight reduction to about a quarter: in this way, the dried sludge is easily stored and movable. Secondly, because the dried sludge has a calorific value comparable to that of coal so that the sludge obtained at the end of the drying process can be reused for energy production. In this regard, in fact, more and more industrial plant managers are interested in making investments to implement new technologies at their plants capable of drying the sludge using either the residual heat present in the process and / or in the waste water or solar energy. in order to produce a dried mud which, thanks to its high calorific value, can represent a precious fuel source.
What are the sludges?
Sludge is the final waste result of wastewater purification.
How much do we produce?
As an order of magnitude, about 1000 t / year are produced for every 15,000 inhabitants, with a dry matter content (dry matter contained in the mixture) of about 22-25%.
Therefore, the treatment plant of a town like Copparo produces a quantity of sludge equal to over 30 “trucks” (1,000 tons / year) for which there is the problem of finding a correct allocation.
2. HOW MUCH WATER IS CONTAINED IN THE SLUDGE?
As highlighted above and trying to ensure the best functioning of the plants and to optimize the costs of transport and subsequent disposal of the sludge produced, the only solution currently feasible is to increase the percentage of dry matter to be removed by reducing the contents. of water present in the mud.
But actually how much water is contained in the sludge?
As we have already seen, a traditional sludge dewatering system is able to significantly reduce the amount of water: starting from values equal to about 2% of dryness characteristic of thickened sludge, we arrive at values equal to about 20-25% and in addition, for dehydrated sludge, with a weight / volume reduction of about 10 times.
But in this mixture there is still more than ¾ of water!
We understand that in terms of disposal costs it is REALLY TO MUCH!
Why, in fact, do we have to transport this quantity and occupy spaces at the final destination with a water content that can be previously eliminated and put back into the liquid circulation?
This is why, for years now, it has been decided to evaporate this superfluous water, drying the shovelable mud that comes from “simple” dehydration systems.
How is sludge dried?
Mainly with heat, but there are several ways to produce it.
In fact, while in the past the dryers were powered simply with heat produced by hydrocarbons or methane, for some time now, all the treatment can be performed with the best renewable source:
THE SUN!
Introducing the drying solar greenhouses in fact allows a significant reduction in the current production of mud, thanks to the decrease in the percentage of water present in the same
3. WHAT IS A SOLAR GREENHOUSE FOR SLUDGE DRYING?
Essentially, it is the action of the Sun (actually solar radiation), combined with the movement of the air, which generates the evaporation of water from the muddy mass.
The structure, very similar to a greenhouse for flower cultivation, helps to “force” a completely natural action. Also the ventilation is always forced to facilitate the speed of moisture transfer to the air. The same, under certain conditions always monitored, is transported outside the greenhouse with or without deodorization treatment depending on its quality.
Pilot plants and real conditions of greenhouses in operation in several parts of the world such as in Germany, show that the treatment is necessary for about 30% of the days of use of the greenhouse.
Obviously, given the more favorable climatic conditions in Italy, even better results are expected.
4. HOW DOES A SOLAR GREENHOUSE FOR SLUDGE DRYING WORK?
The solar drying system is based on the greenhouse effect, the sludge, once dehydrated and brought to a certain percentage of dryness, is distributed inside the greenhouse and mixed periodically with a mechanical device in a completely automatic way.
The sizing of the solar drying treatment clearly requires the climatic data of the site of interest (average temperatures and solar radiation) on which the evaporation capacity of the water inside the greenhouse depends and therefore the drying efficiency and the performance of the treatment .
Essentially a solar drying system consists of the following macro components:
– Waterproof concrete base with a steel greenhouse and polycarbonate roofing,
– Sludge handling system (by means of distributing screws and overhead hoists),
– Air handling system,
– Air treatment system (if any).
5. ODOR EMISSIONS
To date, a problem that limits the implementation of these new sludge drying technologies is related to odor emissions. These systems, in fact, could cause significant odor emissions related to the handling, treatment and storage of sludge which would add up to those normally present in the plant.
The treatment and abatement of odor emissions cannot ignore the knowledge of the processes and specific processing stages from which these emissions are generated. The knowledge of these elements allows to identify the most appropriate ways of controlling, inhibiting and / or mitigating the emission and which consist of:
1. Palliative methods (for example the use of substances with a pleasant smell or coverings of the sources);
2. Preventive methods (preventive reduction of emissions at source by eliminating foul-smelling substances or limiting the conditions that favor their formation and release into the atmosphere);
3. Curative methods (capture and subsequent treatment for purification of emissions).
The first two methods require some planning and management measures which may, at times, be either economically too expensive or not sufficient for the solution of the problem.
The third method, on the other hand, involves the use of appropriate treatment technologies which, by removing the pollutant from the captured air, operate when the emission has already formed. The main technologies for the treatment of odor emissions known to date can be classified in consideration of the principle of action: physical (adsorption), chemical (absorption, thermal oxidation, catalytic oxidation, nebulization, ionization, photo-oxidation and incineration) and biological ( scrubber, biotrickling, biofiltration).
It is therefore of fundamental importance to install a system for the control of odor emissions by means of a perimeter detection probe and to equip the system with a diffusion model that, using software, allows the representation on a map of the actual diffusion “plume”.
The TOM (Total Odor Management) software is a suite for managing and monitoring the odor emissions of a plant. It receives the meteoclimatic data from the control unit and from the analyzers installed on the system and calculates the olfactory impact represented by the odor dispersion plume expressed in olfactory-metric units per cubic meter, using the mathematical model of odor fallout called “AERMOD ”, Typically updated every 5 minutes.
The TOM program resides on the cloud and does not require a dedicated computer; all the calculations are available in real time by accessing the dedicated website with your credentials. The representation of the odor plume, viewable on PC, tablet and smartphone; depict the current olfactory impact or any previous situation.
The olfactory impact is calculated in reverse modeling mode: the software calculates the odor flow value of the sources based on the odor concentration detected by the analyzer positioned downwind, after which it assigns the odor flow value calculated to the sources and calculates it the plume of odor emitted.
For the calibration of the TOM Software, on-site inspections are planned by means of Odor Field Inspection according to the UNI EN 16841: 2017 standard. Odor Field Inspection is used for the validation of odor dispersion models: UNI EN 16841-2: 2017 describes the investigation procedure in the field for the evaluation of the odor plume, in which the assessors are arranged downstream at distances different from the source and provide a sensory response that is compared with the prediction of the mathematical model for the emissive and meteorological conditions recorded at the time of the test.
In order to evaluate the osmogenic impact produced by the emissions conveyed by the solar sludge drying system, it is appropriate to develop a design aimed at sampling, in several cycles, the exhausted air in correspondence with the expulsion duct of the drying plant.
Specifically, for each work cycle of the plant, the air samples will be collected daily for the first three days and more frequently in the following days, until the completion of the mud drying procedure.
The air samples collected in this way will be analyzed in order to determine:
1. the total concentration of odor by dynamic olfactometry according to UNI EN 13725: 2004;
2. Volatile Organic Compounds (VOC) concentrations by thermal desorption and gas chromatographic analysis coupled to mass spectrometry (TDGC / MS);
3. the qualitative and quantitative determination of H2S and NH3 concentrations in emissions with Cairpol sensors.
The samples thus collected will be transported as quickly as possible in isothermal containers in the absence of light to a trusted laboratory for subsequent olfactometric and chemical analyzes.
6. WHAT ADVANTAGES DOES A NATURAL SLUDGE DRYING SYSTEM HAVE?
The solar mud drying technology has the advantage of having low energy consumption and low management and maintenance costs, in the face of the need for large surfaces.
The system differs substantially from other sludge drying techniques due to its very low environmental impact. In fact, the choice of this treatment system was also identified in the light of the better perception by the population of a natural treatment system with a low visual impact.
Obviously, the advantages both from the point of view of management costs and economic savings for the disposal of sludge are considerable, as reported below.
In fact, suppose we want to build a solar drying greenhouse that can enslave a population of about 150,000 – 180,000 PE and which therefore produces approximately 10,000 tons / year of sludge, therefore a production equal to 10 times that of Copparo.
As can be seen from the table, the sludge to be “poured” into the greenhouse is assumed to have an average amount of dry matter equal to 22%. The desired result will be equal to 76%.
This translates into a saving, in terms of evaporated water, of 6,945.1 tons.
From an economic point of view, we are aware that the current price for the disposal of the sludge produced by the purification plant, net of transport, is € 180 / ton.
Therefore, it is possible to make a comparison between the cost of disposal of the sludge produced to date and that produced following the treatment of the sludge inside drying solar greenhouses, taking into account both the cost of disposal and that of the treatment:
As can be seen from the table, the economic savings that would be obtained in terms of sludge disposal, by creating a solar drying greenhouse, is equal to 1,152,367.46 euros per year.
7. HOW IS IT POSSIBLE TO REUSE THE DRIED SLUDGE?
In the near future, we hope that the problem linked to the conversion of a residue into an economic asset of high commercial value will be almost completely eliminated through the adoption of good design and constriction practices which, respecting sustainable value chains, will lead to to the production of easily recyclable and reusable goods. Until then, the greatest commitment will be to consider all our waste, no longer as waste, but as materials to be reused in new applications.
Once dried in a solar greenhouse, the sludge can be used for multiple purposes with a view to a totally green circular economy, including:
– Use in agriculture,
– Use in industry,
– Energy recovery.
A particular application is to produce ecological building bricks starting from the biosolids generated by the sewage sludge. Biosolids are a by-product of the purification process and it is estimated that over 9 million tons of them are produced annually in the European Union alone. As materials that are difficult to reuse, most of these end up accumulated in landfills but, now, a solution has been found that goes well with the principles of the circular economy and sustainability.
The results showed that the addition of biosolids to clay for the production of bricks represents a promising ecological approach useful for reducing greenhouse gas emissions and soil demand. A production of this type would in fact make it possible to greatly reduce the demand for clay and consequently limit the excavations aimed at finding it from virgin soils.
Furthermore, thanks to the higher content of organic substance present in the biosolids, a smaller amount of energy is required to be used in the furnaces.
In particular, the researchers demonstrated that in this way it is possible to save up to 50% of energy with obvious advantages in terms of environmental impact on the part of the brick manufacturing industries. Life Cycle Assessment (LCA) has shown that incorporating biosolids in the production of bricks would lead to a significant reduction in all negative environmental impacts compared to control bricks, with the exception of water consumption.
Not only that, there would also be advantages from a strictly construction point of view since the production of bricks thus conceived, in addition to being less expensive, leads to a lower thermal conductivity, giving the buildings potentially superior environmental performance.
The countless advantages are listed below:
– Reduction of CO2 emissions, due to the reduction of transport times;
– Certainty that all the material (mud) reaches the processing plants;
– Certain processing points;
– Economic development of the territory;
– Social development, through the creation of specialized employment.
8. CURRENT PROJECTS
In light of the problems highlighted above related to the disposal of sewage sludge and with the main purpose of ensuring the best functioning of the plants and optimizing the costs of transport and subsequent disposal of the sludge produced, Acquedotto Pugliese SpA in its capacity as Water Service Manager Integrated of the Puglia Region, it is pursuing the solution of increasing the percentage of dry matter to be removed by decreasing the water content present in the mud through the construction of solar drying greenhouses in three different Apulian cities: Brindisi, Copertino and Casarano.
The aim is to create a complete chain of treatment of the sludge produced by the purification plant extracted from the solar greenhouses with a water content between 20% and 30% (i.e. with a dry content between 70% and 80% ) unlike the current dehydrated which originates mechanically dehydrated sludge with a water content of about 80%.
Solar drying greenhouses, in their future configuration, will consist of a structure of this type:
Il processo seguirà il seguente schema a blocchi.
The structures that will house the entire process are presented as simple greenhouses with a steel supporting structure, polycarbonate cladding and roofing on a concrete slab.
The incoming sludge is discharged to the ground through the use of n. 4 outlets placed in strategic points and, subsequently, through the use of mobile digging cranes, are distributed throughout the plant surface of the greenhouses.
In addition, inside each greenhouse there is the installation of air recirculation and air inlet fans. The flow of air is opposite to the flow of mud.
After the dehydration phase, the mud comes out on the opposite side from where the entrance is; in particular, the dehydrated sludge is led into a hopper from which an auger originates which leads it into a bin, from which it will be collected using heavy vehicles that will lead it to disposal.
The exhausted air will be treated by means of a two-stage horizontal scrubber before being released into the atmosphere.
Here are some renderings representative of Copertino’s project:
9. CONCLUSIONS
The proposed drying system is characterized by its very low environmental impact, as well as by the absence of odor emissions and therefore by the good integration of the work within the urban context of reference.
With the solar drying process, the resulting sludge is dry enough to avoid biological processes that generate odors, but it is still moist enough to avoid the production of dust. Thanks to the porous structure, high evaporation rates and drying efficiency are achieved. The dried sludge produced is stable, with round granules, separated from each other, and easy to handle thanks to the high degree of dryness.
