Potassium recovery from centrate: taking advantage of autotrophic nitrogen removal for multi‐nutrient recovery

Abstract

BACKGROUND: Potassium is a key macronutrient, but little attention has been paid to its recovery from waste streams. This study investigates simultaneous recovery of phosphorus and potassium in the form of potassium struvite (MgKPO 4 ·6H 2 O) from centrate after nitrogen removal by partial nitritation-anammox (PNA). Lab-scale batch experiments to assess the influence of pH, Mg:P ratio and temperature on nutrient removal were conducted on effluent from two PNA reactors fed with centrate from municipal wastewater treatment plants.RESULTS: pH had a strong impact on potassium removal, which increased up to pH 11. At this pH, a product containing 11.4% P, 3.51% N and 4.34% K was obtained. X-ray diffraction confirmed the presence of potassium struvite together with ammonium struvite (MgNH 4 PO 4 .6H 2 O). PNA reduced the alkalinity by 90%, which surpasses the performance of conventional CO 2 stripping by aeration. CONCLUSION: Coupling PNA with struvite precipitation allows for the recovery of N, P and K in a multi-nutrient product and may drastically reduce the need for alkali dosing for pH control.

Recovery of Phosphorus from Waste Water Profiting from Biological Nitrogen Treatment: Upstream, Concomitant or Downstream Precipitation Alternatives

“…Mg precipitates Ca precipitates P precipitation after biological N treatment [73][74][75][76] P precipitation together with biological N treatment [70][71][72] P precipitation before biological N treatment [66][67][68][69] Precipitated-P fertilisation testing [63][64][65] Demonstration & full-scale P recovery experiences [9,28,[61][62] Prediction of P precipitation in waste water treatment systems [55][56][57][58][59][60] P recovery as a resource [10,13,21,[53][54] Formation of P precipitates & deposits in waste water treatment systems [22,[24][25]52] P removal as a pollutant [23,[49][50][51] Chemical properties of MeP: solubility, crystallisation [45][46][47][48] Figure 1. Trend on the number of published papers related to mineral phosphate precipitation from waste water (Source: Web of Science; Accessed: January 10th 2020).…”

“…Previous experiences reporting P precipitation after biological N oxidation or removal are summarised in Table 4 [73][74][75][76]99,124,[205][206][207][208][209][210] (an extended version of this table is provided as Supplementary Material). Biological N treatment based on nitrification involves a reduction in the buffering capacity of the liquid effluent.…”

“…Denitrification and anammox processes counteract this effect, but only partially, as described in Section 4. Thus, downstream precipitation takes advantage of the high decrease in alkalinity commonly achieved through biological treatment [74,76], resulting in a lesser consumption of chemicals to increase the pH value [73,124]. Moreover, the alkaline environment applied during precipitation may favour sanitation of water by enhancing pathogen destruction [99,211].…”

“…Alternatively, addition of Ca(OH) 2 following high-rate biological treatment has also been suggested to raise pH, favouring ammonia stripping and CaP precipitation [212]. [75] Lack of ammonium in the N treated effluents allows raising of pH for precipitating MgP without the risk of favouring ammonia emission while recovering other minerals rather than MAP, such as newberyite [213] (which has a specific P content higher than MAP, see Table 1) and MPP (K-struvite) [75,76]. Potassium, together with N and P, is a key macronutrient for plants typically obtained by mining soluble K-salts (potash) [214,215].…”

“…Yet, there is growing interest in obtaining K from alternative renewable resources, including waste waters. Optimum pH for precipitating MPP is higher than for precipitating MAP, within the range 10.5-11.5 [76,216,217]. Successful attempts for simultaneously precipitating P and K have been reported using raw urine [218,219], digested sewage sludge dewatering centrate [76], digested livestock manure centrate [75], and other waste water streams.…”