Development and modelling of a steel slag filter effluent neutralization process with CO2-enriched air from an upstream bioprocess

Abstract

The main objective of this project was to develop a steel slag filter effluent neutralization process by acidification with CO-enriched air coming from a bioprocess. Sub-objectives were to evaluate the neutralization capacity of different configurations of neutralization units in lab-scale conditions and to propose a design model of steel slag effluent neutralization. Two lab-scale column neutralization units fed with two different types of influent were operated at hydraulic retention time of 10 h. Tested variables were mode of flow (saturated or percolating), type of media (none, gravel, Bionest and AnoxKaldnes K3), type of air (ambient or CO-enriched) and airflow rate. One neutralization field test (saturated and no media, 2000-5000 ppm CO, sequential feeding, hydraulic retention time of 7.8 h) was conducted for 7 days. Lab-scale and field-scale tests resulted in effluent pH of 7.5-9.5 when the aeration rate was sufficiently high. A model was implemented in the PHREEQC software and was based on the carbonate system, CO transfer and calcite precipitation; and was calibrated on ambient air lab tests. The model was validated with CO-enriched air lab and field tests, providing satisfactory validation results over a wide range of CO concentrations. The flow mode had a major impact on CO transfer and hydraulic efficiency, while the type of media had little influence. The flow mode also had a major impact on the calcite surface concentration in the reactor: it was constant in saturated mode and was increasing in percolating mode. Predictions could be made for different steel slag effluent pH and different operation conditions (hydraulic retention time, CO concentration, media and mode of flow). The pH of the steel slag filter effluent and the CO concentration of the enriched air were factors that influenced most the effluent pH of the neutralization process. An increased concentration in CO in the enriched air reduced calcite precipitation and clogging risks. Stoichiometric calculations showed that a typical domestic septic tank effluent with 300 mg/L of biodegradable COD provides enough biological CO for neutralization of a steel slag effluent with pH of 10.5-11.5. A saturated neutralization reactor with no media operated at hydraulic retention time of 10 h and a concentration of 2000 ppm in CO enriched air is recommended for full-scale applications.

Phosphorus Removal and Carbon Dioxide Capture in a Pilot Conventional Septic System Upgraded with a Sidestream Steel Slag Filter

“…The slag filter resulted in a favorable pH above 11, and an effluent o-PO 4 and TP below 0.1 and 0.3 mg P/L, respectively, during the 275 days of the tests. The water quality of the slag filter effluent was consistent with previous observations from the slag filters of the same media [17]. The cumulative P retention of the slag filter increased slowly up to 105 mg TP/kg slag (Figure 4), which is lower than the cumulative removal of 300-1700 mg TP/kg slag observed in six recent slag filter studies for domestic wastewater treatment [6].…”

“…The septic tank effluent o-PO4 and Ca concentrations from this 2015 project were equilibrated with finely-grained hydroxyapatite for pH between 8.3 and 9.0. Results suggest that in a septic tank improved with a sidestream slag filter, phosphorus is removed by hydroxyapatite precipitation as observed in steel slag filters [17]. Note that equilibrium with finely-grained hydroxyapatite with a solubility product of 10 −46 resulted in a realistic o-PO4 concentrations between 0.1 mg/L and 10 mg P/L for pH between 7.5 and 9.0.…”

“…CO 2 flux was calculated assuming steady state conditions in the septic tank exposed to confined air (enriched in CO 2 ). The CO 2 partial pressure was fixed at various values within a range of 2000 to 10,000 ppm, representing small or large concentrations [17] in the confined air of the reactor.…”

“…Active CO 2 sequestration is possible in configurations with a filter sealed with minimum contact with the atmosphere. In such cases, CO 2 -enriched air from an upstream biological reactor can be used for the neutralization of a slag filter effluent [17]. Few researchers measured greenhouse gas emissions or quantified the CO 2 capture of alkaline filters.…”

“…They found that adding oil shale ash to the mesocosms significantly reduced CO 2 emissions compared to peat alone. Bove et al [17] calculated that up to 75% of the CO 2 produced in a secondary treatment was sequestrated by the neutralization of a steel slag filter effluent with CO 2 -enriched air from the secondary treatment.…”