STATE OF THE ART AND POTENTIAL OF THE TECHNOLOGIES FOR PHOSPHORUS RECOVERY FROM SEWAGE SLUDGE

Abstract

Phosphorus is a limited resource and its availability will decrease in the next decades. Therefore, it is important to study viable ways to recover phosphorus before it gets dispersed in the environment in too low concentrations. This study summarizes the potential recovery of phosphorus from wastewater and sludge, framing them in the broader field of phosphorus production and consumption. The regulations in force at European, national and regional level are limited to the reuse of sludge in agriculture, also as material for the production of fertilizers. The main processes/technologies for the recovery of phosphorus from the sludge line of wastewater treatment plants, both from the liquid fraction, from wet sludge, and, finally, from residual ash downstream of incineration processes are described. Finally, this report shows some preliminary data of an ongoing survey on the potential recovery of phosphorus from the sludge produced in the Milan district. The Italian production of sludge is about three million tons per year at an average dry content of 25%. The average concentration of phosphorus in the sewage sludge produced in Lombardy is 2.10% (as Ptot), in line with the value of the 75th percentile of the sample on a national scale. The average ash fraction in the sludge of the same sample is about 30%, leading to an estimate of 7% Ptot in sludge ashes or 16% as P2O5, comparable to phosphate rocks (10.9% Florida; 27.1% Morocco). In Lombardy, almost all wastewater

treatment plants (WWTPs) do not operate biological phosphorus removal; occasional and uncontrolled events can be observed in the largest WWTPs in relation to the variable characteristics of the influent, to the climatic conditions and to the modes of operation. There are two main reasons that make phosphorus recovery less attractive in Italy if compared to other typical situations in Northern Europe. Firstly, in the WWTPs considered, influent P concentration is about 4-5 mg/l, lower than the reference system considered for the assessments in Northern Europe WWTPs, usually about 8-9 mg/l. Consequently, with a limit to the effluent of 1 mg/l, the percentage of phosphorus removal is lower. Secondly, the use of iron and aluminium salts for the chemical precipitation of phosphorus is widely used. This practice makes it more difficult to extract the phosphorus from the ashes, which is bound with iron and aluminium salts. The possibility of recovering sewage sludge products, recently defined by the acronym STRUBIAS (struvite, biochar, ashes from sludges) depends on:

• EU directives and specific national legislation aimed at favouring/encouraging the circular economy and end-ofwaste (materials that lose the status of waste and become marketable products);


• compliance with the technical specifications and regulatory requirements of the recovered product (e.g.: effectiveness of the fertilizing power);


• socio-economic and environmental impacts, measurable with life cycle analysis of the product;


• technical skills, better exploitation and use of existing infrastructures and business model of recovery processes and technologies;


• market, which depends, as well as price, on the acceptability of products recovered by end

The minimum cost of recovery for some processes is 2-3 €/kg of P recovered compared to about 1-1.5 €/kg of P from classical mineral sources. Therefore, today a recovery P market cannot be achieved without incentive actions that act on price, quality requirements and, finally, on the acceptance of transformers/users.