Method for recycling sewage sludge from the treatment of civil wastewater to agriculture using a mixture containing sodium chlorite with the formation of chlorine dioxide

Abstract

The present invention relates to a method for treating sewage sludge from the treatment of civil wastewater, comprising the following steps: - mixing the sludge with a quantity of deionised water between 5 and 20 times the weight of said sludge; - adding an acid solution, until a pH of less than 2 is reached; adding NaCIO2 in such a quantity that the amount of acid is in excess of the amount of NaCIO2, sodium chlorite reacting with hydrochloric acid to give chlorine dioxide, sodium chloride and water, according to the reaction: 5NaCIO2 + 4HCI 4CIO2 + 5NaCI + 2H2O - allowing to react for at least 1 hour.

Description

[0001] Method for recycling sewage sludge from the treatment of civil wastewater to agriculture using a mixture containing sodium chlorite with the formation of chlorine dioxide

[0002] The present invention relates to a method for recycling sewage sludge from the treatment of civil wastewater to agriculture, after treating the sludge with a mixture containing sodium chlorite, with the formation of chlorine dioxide.

[0003] As is well known, sewage sludge from the treatment of civil wastewater constitutes a major environmental problem, because, once produced, it must be disposed of in landfills. About 3 tons of sewage sludge (data from ISPRA - Higher Institute for Environmental Protection and Research) are produced in Italy alone every year, and 56% must be disposed of in landfills, with consequent evident problems connected to the population’s opposition to this type of solution, which is particularly understandable if one considers that the sludge currently produced by many civil wastewater treatment plants emits foul odours and has high levels of bacterial colonies, above all of salmonella and Escherichia coli.

[0004] The remaining amount of sewage sludge from the treatment of civil wastewater, which is not disposed of in landfills, is sent to waste-to-energy plants and burned. Although the waste-to-energy process enables energy to be produced from the combustion of sludge, this process likewise entails environmental problems due to the production of pollutants that are released into the atmosphere, besides being costly for businesses.

[0005] In recent years, growing attention has been paid to the problems connected to environmental pollution. In this context, an economic system called circular economy has been conceived; it aims to ensure development compatible with eco- sustainability through a model of production and consumption attentive to reducing the waste of natural resources and consisting in the sharing, reuse, repair and recycling of existing materials and products for as long as possible.

[0006] Therefore, in the specific sector of sewage sludge from the treatment of civil wastewater, there exists a need to develop new sludge treatment processes that may enable a reduction of waste through maximisation of sludge recycling and reuse. This need is satisfied by the method for recycling sewage sludge from the treatment of civil wastewater to agriculture according to the present invention, which offers, moreover, further advantages that will become clear hereafter.

[0007] The solution according to the present invention fits into this context; it aims to provide a method for recycling sewage sludge from the treatment of civil wastewater to agriculture, following treatment with sodium chlorite.

[0008] In particular, the method according to the present invention proposes using sludge in the agricultural sector, in accordance with the following requirements:

[0009] - the fields for which the previously treated sewage sludge is intended, must not be a source of foul odours for the surrounding population;

[0010] - the concentration of metals in the sludge must be controlled in order to comply with the limits set by law;

[0011] - the concentration of hydrocarbons in the sludge must be controlled in order to comply with the limits set by law;

[0012] - the number of bacterial colonies present in the sludge must be below with the limits set by law; in particular, two types of bacteria, salmonella and Escherichia coli, must be kept under control, as must helminth eggs.

[0013] These and other results are obtained according to the present invention by proposing a method for recycling sewage sludge from the treatment of civil wastewater to agriculture, after treating the sludge with a mixture containing sodium chlorite, with formation of chlorine dioxide.

[0014] The aim of the present invention is thus to provide a method for treating sewage sludge from the treatment of civil wastewater which makes it possible to overcome the limits of the methods according to the prior art and to obtain the previously described technical results.

[0015] A further aim of the invention is that said treatment method may be carried out with substantially low costs.

[0016] Yet another aim of the invention is to propose a method for treating sewage sludge from the treatment of civil wastewater that is simple, safe and reliable.

[0017] Therefore, a specific object of the present invention is a method for treating sewage sludge from the treatment of civil wastewater, comprising the following steps:

[0018] - mixing the sludge with a quantity of deionised water between 5 and 20 times the weight of said sludge; - adding an acid solution until a pH of less than 2 is reached;

[0019] - adding NaCIO2 in such a quantity that the amount of acid is in excess of the amount of NaCIO2, sodium chlorite reacting with hydrochloric acid to give chlorine dioxide, sodium chloride and water, according to the reaction:

[0020] 5NaCIO2+ 4HCI 4CIO2+ 5NaCI + 2H2O

[0021] - allowing to react for at least 1 hour and preferably for at least 2 hours.

[0022] According to the invention, said acid solution is added until pH values less than or equal to 1 are obtained, and more preferably equal to 0.5.

[0023] Preferably, according to the invention, said acid can be chosen from HCI and H2SO4 and more preferably it is HCI.

[0024] In particular, again according to the invention, the added amount of said sodium chlorite is such as to obtain an excess of at least 300% of the amount of acid over the amount of NaCIO2.

[0025] Moreover, again according to the invention, said NaCIO2can be added in solution or powder form, preferably in a 25% aqueous solution.

[0026] Finally, according to the present invention, said 25% solution of NaCIO2is added in an amount of at least 0.5g per gram of sludge, considered dry, corresponding to about 0.12g per gram of wet sludge, with a degree of moisture corresponding to a value of 76.4%, which corresponds to the average moisture value of sewage sludge from the treatment of civil wastewater.

[0027] The invention will be described below by way of non-limiting illustration, with particular reference to several illustrative examples.

[0028] In particular, according to the method for recycling sewage sludge from the treatment of civil wastewater to agriculture, it is proposed to add solutions containing sodium chlorite to the sludge, which allows reducing the number of bacterial colonies present and eliminating unpleasant odours. In fact, sodium chlorite added to sludge reacts and forms chlorine dioxide, as will be described below, chlorine dioxide being an effective bactericidal / germicidal agent.

[0029] Furthermore, chlorine dioxide is also an oxidising agent, capable of causing the oxidation of some organic compounds present in the sludge, thus leading to a reduction or to the disappearance of the unpleasant odours caused by these compounds.

[0030] In particular, according to the present invention, the hydrochloric acid present in the sludge reacts with sodium chlorite to give CIO2(chlorine dioxide), NaCI (sodium chloride) and water, according to the reaction:

[0031] 5NaCIO2+ 4HCI 4CIO2+ 5NaCI + 2H2O

[0032] Theoretically, in order to react 5m L of a 25% solution of sodium chlorite it is necessary to add 0.12mL of a solution of 33% HCI by weight. In practice, however it is necessary to add an excess of HCI to reduce the formation of undesirable byproducts such as chlorates.

[0033] The necessary pH value must be very acidic (with a pH of less than or equal to 2, preferably a pH equal to 0.5). This pH range enables the maximum conversion of sodium chlorite to chlorine dioxide (CIO2) to be obtained.

[0034] Furthermore, examples of how bacteria normally present in sludge, such as Escherichia Coli and also salmonella, manage to survive at extreme pHs have been reported in the scientific literature. In fact, extreme pH conditions do not suffice on their own to eliminate the bacterial load, and hence the unpleasant odours. In these cases, the bacteria succeed in adapting to environments having extreme pHs, so in order to achieve control of their proliferation, as well as of the odours connected to them, it is necessary to generate chlorine dioxide CIO2, which has excellent disinfectant activity, as it is capable of destroying the cell membrane or DNA of bacteria, thereby killing them.

[0035] According to the invention it is possible to use hydrochloric acid at different concentrations, or other types of acids, such as, for example, sulphuric acid.

[0036] As regards the maximum amount of acid to be added, there is no actual limit. The important thing is for the pH to remain less than or equal to two and preferably less than or equal to one.

[0037] Consequently, the amount of hydrochloric acid must be a good deal above the stoichiometric ratio determined by the reaction indicated previously, and it is preferably necessary to work with an excess of hydrochloric acid equal to at least 300%.

[0038] If acids other than hydrochloric acid are used, the amount of acid to be used and the type of by-products obtained will change. However, it is always envisaged that chlorine dioxide (CIO2), which is a strong bactericidal agent, will be formed.

[0039] As regards sodium chlorite, it can be used at various concentrations or even in powder (after putting in place the necessary measures to counter the safety problems due to the high corrosiveness of sodium chlorite powder) and reacted with a suitable amount of acid. As regards the ratio between the amount of sludge and the amount of sodium chlorite to be added, it can be varied according to needs. In particular, in order to reduce the costs due to the purchase of raw materials, it is possible to use the largest amount of sludge with the smallest volume possible of sodium chlorite solution, that is, making reference to the amount of dry sludge to eliminate margins of approximation tied to the degree of moisture of the sludge, it is necessary to add at least 0.5g of a 25% aqueous solution of sodium chlorite per gram of dry sludge. In general, given that sewage sludge from the treatment of civil wastewater has, on average, a moisture content of 76.4%, in a 50g sample of wet sludge, the dry portion corresponds to about 12g of dry sludge. Therefore, using a 25% solution of sodium chlorite to treat sludge with a degree of moisture corresponding to the average value 76.4%, the minimum amount of solution to be added is about 0.12g of 25% solution of sodium chlorite per gram of wet sludge.

[0040] From of an industrial viewpoint, this reaction can be carried out using suitable chlorine dioxide generators, already existing on the market, to be applied to the tank containing the sludge to be treated, since these devices enable an optimal, more uniform generation of chlorine dioxide compared to the simple manual addition of reagents to the sludge suspension.

[0041] Example 1 .

[0042] 53.7 g of wet sludge from a sewage treatment plant were introduced into a beaker containing 400 mL of deionised water and stirred. After about 20 minutes, 10 mL of a 33% solution of HCI were added under stirring. The pH was measured and showed to be between one and two. Then 6 mL of a 25% solution of sodium chlorite were added and the reaction continued for about two hours. Once the reaction had ended, the suspension was placed in a receptacle. Example 2.

[0043] 10 g of dry sludge from a sewage treatment plant were introduced into a beaker containing 200 mL of deionised water. The rotation speed was about 700 rpm. Then 5 mL of a 33% solution of HCI and 5 mL of a 25% solution of sodium chlorite were added. After the addition of HCI, the pH of the solution was measured and showed to be between one and two. The reaction continued for about two hours. Once the reaction had ended, the suspension was placed in a receptacle. Example 3. beaker containing 400 mL of deionised water and stirred. After about 20 minutes, 5 mL of a 30% solution of H2SO4 were added under stirring. The pH was measured and showed to be about 1. Then 6 mL of a 25% solution of sodium chlorite were added and the reaction continued for about two hours. Once the reaction had ended, the suspension was placed in a receptacle.

[0044] The suspensions obtained from each of the examples 1 -3 were analysed both before and after the treatment. In all cases, a considerable decrease in the bacterial load was observed, since the salmonella was completely eliminated and a reduction in the number of colonies of Escherichia Coli was observed, with the disappearance of the unpleasant odours. Notwithstanding that the pH remained above the ideal value (0.5), a considerable effectiveness of the product was observed.

[0045] The present invention has been described by way of non-limiting illustration, according to the preferred embodiments thereof, but it is understood that variations and / or modifications may be introduced by the person skilled in the art without going beyond the relevant scope of protection, as defined by the appended claims.

Claims

CLAIMS1 ) Method for treating sewage sludge from the treatment of civil wastewater, comprising the following steps:- mixing the sludge with a quantity of deionised water between 5 and 20 times the weight of said sludge;- adding an acid solution, until a pH of less than or equal to 2 is reached;- adding NaCIO2in such a quantity that the amount of acid is in excess of the amount of NaCIO2, sodium chlorite reacting with hydrochloric acid to give chlorine dioxide, sodium chloride and water, according to the reaction:5NaCIO2+ 4HCI 4CIO2+ 5NaCI + 2H2O- allowing to react for at least 1 hour.2) Method for treating sewage sludge from the treatment of civil wastewater according to claim 1 , characterised in that said acid solution is added until pH values less than or equal to 1 are obtained.3) Method for treating sewage sludge from the treatment of civil wastewater according to claim 2, characterised in that said acid solution is added until pH values of 0.5 are obtained.4) Method for treating sewage sludge from the treatment of civil wastewater according to any one of the preceding claims, characterised in that said acid is chosen from HCI and H2SO4.5) Method for treating sewage sludge from the treatment of civil wastewater according to claim 4, characterised in that said acid is HCI.6) Method for treating sewage sludge from the treatment of civil wastewater according to any one of the preceding claims, characterised in that the added amount of said sodium chlorite is such as to obtain an excess of at least 300% of the amount of acid over the amount of NaCIO2.7) Method for treating sewage sludge from the treatment of civil wastewater according to any one of the preceding claims, characterised in that said NaCIO2is added in solution or powder form.8) Method for treating sewage sludge from the treatment of civil wastewater according to claim 7, characterised in that said NaCIO2is added as a 25% aqueous solution.9) Method for treating sewage sludge from the treatment of civil wastewater according to claim 8, characterised in that said 25% solution of NaCIO2is added inan amount of at least 0.5g per gram of sludge, considered dry.10) Method for treating sewage sludge from the treatment of civil wastewater according to any one of the preceding claims, characterised in that, after the addition of NaCIO2, the mixture is left to react for at least 2 hours.