Enhancing sludge dewaterability and phosphate removal through a novel chemical dosing strategy using ferric chloride and hydrogen peroxide

Abstract

In this study, we replicated full‐scale centrifuge dewatering utilized in water resource recovery facilities (WRRFs) by using the Higgins modified centrifuge technique and demonstrated that analogous cake solid content and centrate suspended solids were attainable while applying a lower polymer dosage. Furthermore, we demonstrated a dramatic reduction in the concentration of phosphate (P) in anaerobically digested sludge (ADS) under various reaction conditions. H2O2 was employed to convert embedded iron in ADS, in the form of FeS, to Fe (II) and Fe (III), which subsequently reacted to precipitate phosphate compounds, dropping the in situ P concentration by nearly 50%. Adding ferric chloride (220 mg/L) in ADS enhanced the P‐removal to more than 80%. Finally, simultaneous dosing of Fe and H2O2 boosted P‐removal efficiency to higher than 90%. The role of Fe in strengthening the flocs and increasing the dewaterability was also substantiated by demonstrating a 2% growth in the cake solid content when ADS was conditioned with Fe + H2O2 preceding polymer treatment. The outcome of this work confirms that a deeper understanding of centrifuge operational parameters and physico‐chemical properties of wastewater sludge would result in improved performance of municipal WRRFs. Practitioner points Dosing hydrogen peroxide effectively converted iron embedded in sludge from Fe (II) to Fe (III). Simultaneous dosing of iron and hydrogen peroxide boosted P removal efficiency. The role of iron in strengthening flocs and enhancing dewaterability was observed, as it increased cake solid content in centrifuged sludge. An advanced bench‐scale test protocol was employed to optimize polymer dose, simultaneously reducing polymer consumption while maximizing cake solid content and centrate quality.

Behavior of Sludge Dewaterability and Nutrient Contents after Treatment with Cellulose-Based Flocculants with Combined PTS and Catalytic Behavior of Sludge towards Tetracycline Degradation

“…Different treatment techniques (crystallization, precipitation, ion exchange, etc.) have been used for inorganic phosphorus removal [21][22][23] but traditional techniques are not well-suited for organic phosphates due to their unique structures. Natural polymers for phosphorus removal have not been extensively studied, but it was shown that adding a starch-based flocculant with FeCl 3 may save one-third of the total cost of pre-treatment while 90% of total phosphorus was reduced [22,24].…”

Model‐based investigation of the chemical phosphorus removal potential of the peroxide regenerated iron‐sulfide control technology

“…In order to reprecipitate phosphorus, an oxidizing agent such as hydrogen peroxide (H 2 O 2 ) is used to dissociate FeS. With partial oxidation, the free ferrous iron would react with orthophosphates to precipitate as vivianite, and with complete oxidation, the free ferrous iron is converted to ferric iron, producing HFOs and eventually removing orthophosphates from the liquid phase (Ghodsi et al, 2021). The application of H 2 O 2 in peroxide regenerated iron-sulfide control (PRI-SC) was described by Walton et al (2004) where H 2 O 2 is fed in the collections system at optimized rates to raw wastewaters freeing ferrous iron to oxidize spent iron (FeS), making that iron available to control sulfide generated downstream.…”

“…While phosphorus removal can be realized through both biological and chemical means, meeting such limits can be achieved through some level of chemical addition such as alum (e.g., Al 2 (SO 4 ) 3 .14H 2 O), ferric iron salts (e.g., FeCl 3 .6H 2 O), ferrous iron salts (e.g., Fe SO 4 .7H 2 O, FeCl 2 ), and in some cases calcium salts or lime (Conidi & Parker, 2016; Thistleton et al, 2002). Chemical phosphorus removal is recognized as a practical cost‐effective method compared to crystallization in fluidized bed pellet reactors or magnetic separation, with iron salts being more preferable and widely used considering that they are less expensive compared with aluminum salts, improve sludge dewaterability, limit corrosion, and minimize odor control issues arising from noxious hydrogen sulfide emissions (Ghodsi et al, 2021; van van Starkenburg et al, 1993; Walton et al, 2003).…”

“…When applied to facility influents for chemically enhanced primary treatment (CEPT), H 2 O 2 is fed at higher rates to produce ferric iron, which in return enhances solid–liquid separation in primary clarifiers. Additionally, Ghodsi et al (2021) demonstrated the feasibility of phosphorus removal using H 2 O 2 by utilizing the already existing FeS in the anaerobically digested sludge (ADS) to free ferrous iron and oxidize it to ferric iron, realizing a 50% removal efficiency by dosing H 2 O 2 solely and a 90% removal efficiency with dosing H 2 O 2 and augmenting the iron by adding ferric chloride. Following the same concept, H 2 O 2 could be applied downstream of the final clarification process to free and oxidize ferrous iron, subsequently decreasing the required ferric dose for phosphorus removal.…”