Peracetic acid for conditioning of municipal wastewater sludge: Hygienization, odor control, and fertilizing properties

Waste Management 102 (2020) 371–379

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Peracetic acid for conditioning of municipal wastewater sludge: Hygienization, odor control, and fertilizing properties Tero Luukkonen a,⇑, Hanna Prokkola b, Simo O. Pehkonen c a

Fibre and Particle Engineering Research Unit, University of Oulu, Finland Research Unit of Sustainable Chemistry, University of Oulu, Finland c H2OPrima, Vihurintie 11, FI-70780 Kuopio, Finland b

a r t i c l e

i n f o

Article history: Received 10 August 2019 Revised 5 October 2019 Accepted 4 November 2019

Keywords: Biosolids Disinfection Fertilizer Peracetic acid Odor control

a b s t r a c t Peracetic acid (PAA) is an environmentally friendly disinfectant and oxidizer used in several water and wastewater treatment applications. In the present study, PAA was utilized for the conditioning of municipal wastewater sludge before thickening and dewatering. It was shown that PAA can effectively prevent odor formation (i.e., H2S and NH3) and provide hygienization (using E. coli and Salmonella as indicators). Phytotoxicity can be prevented by controlling the amount PAA-conditioned sludge that is mixed in the soil to be fertilized. The required PAA dose for hygienization was relatively high (480 mg 100% PAA per L sludge) but the results indicated that other sludge stabilization processes are not necessarily required. Therefore, the proposed process involving PAA could be feasible in cases where limited land area is available for sludge processing or quick conditioning of sludge is required. Ó 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction Municipal wastewater sludge generation in the EU is more than 10 Mt as dry matter annually (Bianchini et al., 2016). Biological sewage sludge, which is also known as biosolids after stabilizing treatment steps (Collivignarelli et al., 2019a), contains significant nutrient resources: typically approximately 3.3 and 2.3% nitrogen and phosphorus (of dry solids), respectively (Tchobanoglous, Burton and Stensel, 2004). Especially phosphorus utilization from sludge is beneficial as phosphate rock deposits are depleting globally, and thus it has been listed as a critical raw material by the EU (Commission’s Communication, 2017). Sludge contains also other organic matter (Wijesekara et al., 2017) that would be beneficial for soils since there has been a decline in the soil carbon content (Heikkinen et al., 2013). Consequently, the utilization of sludge as a fertilizer or soil improver is important, and thus it is encouraged by the Sewage Sludge Directive in the EU (Council Directive 86/278/EEC, 1986). Concentration of seven metals (Cu, Cr, Ni, Pb, Zn, Cd, and Hg) in the sludge to be utilized as fertilizer or soil conditioner are controlled by the EU legislation (Council Directive 86/278/EEC, 1986). However, there have been increasing concerns about other potentially toxic elements, organic micropollutants, and microplastics in sludge. Regarding the potentially toxic elements, ⇑ Corresponding author. E-mail address: [email protected] (T. Luukkonen).

it was recently shown that the repeated amendment of agricultural soils with sewage sludge for 15 years did not accumulate Cu, Cr, Ni, Pb, Zn, Cd, or Hg over the legislative limit values but some increase in unregulated metal(loid)s (As, Co, Sb, Ag, Se, and Mn) was observed (Marguí et al., 2016). Degradation of 72 different pharmaceuticals and personal care products in sludge-soil mixtures showed that their half-lifes varied from approximately 0.5 to 10 years, while some (such as diphenhydramine, fluoxetine, thiabendazole and triclocarban) showed no degradation over the three years observation period (Walters et al., 2010). However, the accumulation of micropollutants or their transformation products (total 141) to vegetables was shown to be low (Sabourin et al., 2012). Nevertheless, even though there is no clear evidence about the harmfulness of the sludge land-application, the low public acceptance of sludge utilization can become an obstacle (Oberg (Öberg) and Mason-Renton, 2018). Typically, the sludge treatment process consists of thickening, conditioning, dewatering, and stabilization stages, for instance, but the order of the aforementioned processes may vary. Thickening and dewatering are used for the sludge volume reduction, whereas stabilization aims for the reduction of pathogen numbers, odor elimination, and putrefaction inhibition. Possible options for stabilization are, for instance, alkaline treatment (i.e., increase of pH to 12 or higher), anaerobic or aerobic digestion, or composting (Tchobanoglous, Burton and Stensel, 2004). In addition, some chemical conditioning methods can be used: for instance, treatment with sulfuric acid and hydrogen peroxide (at pH of 3–5) to

https://doi.org/10.1016/j.wasman.2019.11.004 0956-053X/Ó 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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induce Fenton reactions with Fe2+ present in sludge (Schaum et al., 2008). One alternative sludge conditioning chemical could be peracetic acid (PAA, CH3COOOH). PAA is an environmentally friendly water and wastewater disinfectant and an oxidizer with a low tendency to form harmful by-products (Domínguez Henao et al., 2018a,b). It has been used at full scale or studied for the disinfection of municipal wastewaters, sewer overflows, industrial effluents, potable water, ballast water, aquaculture systems, and for the oxidation of recalcitrant organic compounds in advanced oxidation processes (Liu et al., 2016; Luukkonen and Pehkonen, 2017). For sludge treatment, the research involving PAA is relatively scarce. The inactivation of Salmonella and beef tapeworm (Taenia saginata) in sludge has been reported to occur at PAA doses of 250–1000 mg/L (as 100% PAA) (Baldry and Fraser, 1988; Fraser, Godfree and Jones, 1984). PAA treatment is beneficial before anaerobic digestion, as it can improve organic matter solubilization and sludge disintegration leading to increased biogas production (Appels et al., 2011; Sun et al., 2018; Zhou et al., 2017). Furthermore, pre-oxidation of sludge with PAA decreased the sludge volume and improved dewatering due to the degradation of extracellular polymeric substances (Zhang et al., 2016). Sludge volume decrease with advanced chemical, biological, thermal, or electrochemical techniques can have a significant impact on the sludge treatment costs (Collivignarelli et al., 2019b). It was recently estimated that if such methods are applied to larger than 500,0000 population equivalent wastewater treatment plants in the EU, that alone could decrease sludge amount (as mass of dry matter) by approximately 25% (Collivignarelli et al., 2019b). Furthermore, PAA can be applied to treat biological foaming (also known as sludge bulking or swelling) in an active sludge process (Maunuksela and McIntyre, 2000). The foaming is caused by the presence of filamentous bacteria or extracellular polymeric substance, of which the latter was found to correlate well with the aeration/non-aeration regimes and temperature (Collivignarelli et al., 2017). The effect of PAA on the prevention of foaming is caused by the oxidation of extracellular polymeric substances and the destruction of flocs held together by mycelium (Maunuksela and McIntyre, 2000). In the present study, PAA was used for the conditioning of sludge collected from primary sedimentation and activated sludge process (i.e., before thickening or dewatering). After the PAAtreatment, sludge was centrifuged in laboratory and mixed with low-nutrient soil. The goals of the study were to: (1) find adequate PAA dose for sludge hygienization; (2) determine the amount that PAA-treated sludge can be mixed with soil without causing a phytotoxic effect (i.e., plant growth prevention); and (3) observe the effect of PAA-treatment on the macro nutrient levels in the sludge-soil mixtures.

2. Materials and methods 2.1. Sludge sampling Sludge samples were collected from the Taskila (Oulu, Finland) municipal wastewater treatment plant (WWTP) with 174 200 population equivalent capacity and approximately 50,000 m3/d discharge. The main unit processes of the Taskila WWTP and the sludge sampling point are shown in Fig. 1. Sludge samples were collected on seven occasions during a winter season (November– January). Sludge retention time (SRT) during the sampling was approximately 5 d. Sludge treatment experiments were performed on the same day as the sampling was performed. Typical properties of sludge after sampling are shown in Table 1.

2.2. Sludge conditioning experiments Sludge samples were homogenized with an overhead stirrer using 500 rpm mixing speed for 30 mins. Then, commercially available PAA solution (tradename ProxitaneÒ WW-12, composition as w/w: 12% PAA, 20% hydrogen peroxide, 20% acetic acid, and 0.5–1% sulfuric acid, Solvay, Finland) was dosed into 1.0 L sludge samples at 0, 60, 120, 240, or 480 mg of 100% PAA/L of sludge and mixed for 15 mins (500 rpm). The PAA doses correspond to ratios of 0, 1/2000, 1/1000, 1/500, and 1/250 as kg PAA solution/kg of sludge, respectively, or approximately 0, 1/108, 1/54, 1/27, and 1/14 as kg PAA solution/kg dry matter in sludge, respectively. After the PAA dosing, the sludge samples were centrifuged (using Jouan C412 centrifuge, 2000 rpm, 5 min) to simulate thickening/dewatering. Centrifuging increased the dry matter content of the sludge from an average of 5.4 to 9.6%. Homogenized centrifuged sludge was mixed to low-nutrient soil at 1, 5, or 10 vol%. The low-nutriend soil was prepared by mixing 20 wt% of quartz sand and 80 wt% of commercial soil product (Kekkilä Viljelyseos, Finland, which contains sphagnum peat and sand, has pH of 6.0, conductivity of 20 mS/m, water-soluble nitrogen, phosphorus, and potassium of 300, 70, and 700 mg/kg dry solids, respectively). 2.3. Analysis and calculation methods Conductivity and pH were measured from sludge samples before dewatering using a Hach-Lange HQ40D multi-parameter meter with SPC401 and PHC101 electrodes, respectively. Reduction-oxidation (redox) potential of sludge samples before centrifuging was measured with an EZ-7200 multiparameter probe (EZDO, Taiwan) equipped with a Pt test electrode and a Ag/AgCl reference electrode. Conductivity and pH of sludge-soil mixtures were measured according to standard methods (SFS-EN 13038: en, 2011; SFS-EN 13037:en, 2011): measurements were conducted using a sludge-soil mixture to distilled water ratio of 1:2.5 (V/V). The CO2 production of sludge-soil mixtures (containing 0, 1, 5, or 10 vol% of sludge treated with 0, 120, or 240 mg/L of PAA) was measured by placing the sample in a closed bottle, incubating at +37 °C for 24 h, and determining the gas evolution with an infrared analyzer (Itävaara et al., 2010). The CO2 evolution rate (mg CO2-C/g VS/d) can be calculated with Eq. (1):

CO2 ev olution rate ¼

V p  CO2  M C  t  T NTP T  V NTP  mn  kan  VSn  tx

ð1Þ

where Vp = volume of the test bottle (mL), CO2 = CO2 concentration (%/100), MC = atomic weight of carbon (12.01 g/mol), t = time (24 h), TNTP = temperature at the NTP conditions (273.15 K), T = temperature (K), VNTP = volume of ideal gas at the NTP conditions (22.42 L/mol), mn = weight of the sample (g), kan = dry weight of the sample (%/100), VSn = volatile solids in the sample (%/100), t = time from the beginning of the measurement (h). Biological oxygen demand (BOD) with an allylthiourea addition was measured using the OxiTop manometric respirometric method under the OECD 301F standard conditions. The BOD measurements were conducted from sludge samples before centrifuging. Phytotoxicity (i.e., plant growth prevention) was measured according to the method by Itävaara et al. (2006). In the test, 100 cress (Lepidium sativum) seeds were planted to the sludge-soil mixtures (similar amount of sludge and PAA as with the CO2 production experiment) and to a reference soil, watered, and allowed to grow for 14 d. Then, seedlings were cut, counted, and weighed after drying at 70 °C for 1 d. The growth index (Eq. (2)) less than 80% indicates phytotoxicity. Growth index ¼

weight of seedlings in sludge  soil mixture ðgÞ  100% weight of seedlings in reference soil ðgÞ ð2Þ

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Fig. 1. Schematic presentation of the main unit processes of the Taskila wastewater treatment plant and the sludge sampling point.

Table 1 Typical properties of the sampled sludge. Property

Value

pH Conductivity Redox potential Total solids Biological oxygen demand (30 d) Temperature

6.72 ± 0.10 2390 ± 500 lS/cm 171 ± 18 mV 5.4 ± 0.6% 3500 ± 80 mg/L 8–10 °C

The PAA concentration during the decomposition experiment was measured with a pre-calibrated handheld photometer (Chemetrics, V2000) and the K-7913 cuvettes for PAA concentration (Chemetrics). The analysis is based on the addition of an excess of potassium iodide, which reacts with PAA to form iodine. Iodine then oxidizes N,N-diethyl-pphenylenediamine into a pink-colored species from which absorbance is determined. The method is suitable for the 0–5.00 mg/L PAA range and thus the samples were appropriately diluted before measurement using distilled water. The values of the first order kinetic rate law (Eq. (3)) parameters were obtained with the non-linear regression using the Microsoft Excel Solver tool (GRG nonlinear).

C ¼ ðC 0  DÞ  ekt

emission spectrometer (Thermo Electron IRIS Intrepid II XDL Duo) according to the standard (SFS-EN ISO 11885, 2009). Potentially toxic elements were analyzed as total concentrations from solid samples (sludge-soil mixtures) by first dissolving the sample with a microwave-assisted acid digestion method (U.S. EPA, 2007) and then quantifying the metal(loid) concentrations with an optical emission spectrometer similarly as described above. Total solids content of sludge samples was determined by evaporating at 105 °C until a constant weight was achieved. 2.4. Odor control experiment Septic-condition sludge was placed in airtight glass bottles (volume of sludge was 200 mL). Samples were mixed well, the bottle was opened, and after 10 s, gaseous H2S and NH3 were measured from the opening with a Dräger X-am 5000 gas detector equipped with H2S and NH3 sensors (DrägerSensor XXS H2S and XXS NH3 with measuring ranges of 0–200 ppm and 0–300 ppm, respectively). Redox potential was also measured at the same time. PAA was dosed to samples with incremental increases (with 10 s effective mixing in a closed bottle between each dose), until zero reading of H2S was reached.

ð3Þ 3. Results and discussion

where C = PAA concentration at time t (mg/L), C0 = initial PAA concentration (mg/L), D = initial PAA demand (mg/L), k = rate constant (mg/(L  min)), and t = time (min). E. coli and Salmonella were determined from the PAA-treated sludge samples before centrifuging by applying the NMKL 125 (NMKL 125, 2005) and the VIDASÒ (Bird et al., 2013) methods, respectively. For nutrient (NH+4, NO 3 , P, Ca, K, Mg, and S) analyses, sludge-soil samples (similar amount of sludge and PAA as with the CO2 production experiment) were air-dried, passed through a 2-mm sieve, and extracted with a 0.5 M ammonium acetate and 0.5 M acetic acid solution (pH = 4.65) for 1 h using a 1:10 (V/V) sludge-soil to solution ratio (Vuorinen and Mäkitie, 1955). NH+4 and NO 3 were determined by using a flow injection analyser (Fiastar 5000, Foss-Tecator) according to standard methods (SFS-EN ISO 13395, 1996; SFS-EN ISO 11732, 2005). P was determined colorimetrically by a molybdenum blue method with stannous chloride as the reducing agent. Ca, K, Mg, and S were determined with an optical

3.1. pH, conductivity, and redox potential of sludge and sludge-soil mixtures The sludge pH decreased approximately 0.004 units per 1 mg/L of dosed PAA and, consequently, the largest applied PAA dose (480 mg/L) decreased pH by approximately 1.7 units (Fig. 2A). As a comparison, in secondary or tertiary effluents, PAA has been reported to decrease pH by 0.02–0.03 unit per 1 mg/L PAA (Cavallini et al., 2013; Koivunen and Heinonen-Tanski, 2005). The clearly lower impact of PAA on the sludge pH could be caused by a higher buffering capacity, that is, ammonium, carbonate, and volatile fatty acids (e.g., acetic, propanoic, and butyric acids) equilibrium (Procházka et al., 2012). When PAA-conditioned and centrifuged sludge was mixed with soil, pH of the mixture remained constant 6.2–6.3 regardless of the sludge amount (Fig. 2B). The conductivity of the sludge increased as a function of the PAA dose due to the introduction of hydroxyl (OH) and acetate

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Fig. 2. pH of (A) sludge before dewatering and (B) sludge-soil mixtures. Conductivity of (C) sludge before dewatering and (D) sludge-soil mixtures. (E) Redox potential of sludge before dewatering.

(CH3COO) ions to the sludge (Fig. 2C). When PAA-conditioned and centrifuged sludge was mixed with soil, the conductivity of the mixture increases linearly from 4 to 6 mS/cm at sludge amounts of 0–10 vol% of soil, respectively (Fig. 2D). The sludge contains several water-soluble ionic species affecting the conductivity when mixed into the soil but also the solubilization of sludge organic matter by PAA (Zhang et al., 2016) could have contributed to the increased conductivity. The conductivity of soil between 2.5 and 10 mS/cm is classified as ‘‘high” in the Finnish recommendations for agricultural soils (Eurofins Viljavuuspalvelu Oy, 2019). When conductivity of soil is >10 mS/cm, it can affect water balance of crops detrimentally and actions should be taken to decrease it (Eurofins Viljavuuspalvelu Oy, 2019). However, it is typical that the conductivity of soil is temporarily high after applying a fertilizer. Redox potential of sludge increased steeply as PAA was dosed up to 240 mg/L (Fig. 2E). The increase is a result of PAA and hydrogen peroxide decomposition and a subsequent release of dissolved oxygen. When the dose of PAA is further increased, redox potential levels off at approximately +100 mV. The stabilization of redox

potential might indicate that the sludge sample becomes temporarily saturated with dissolved oxygen (Luukkonen et al., 2014). The redox potential behavior has important implications for odor control. The initial redox potential (171 mV) of sludge indicated an anoxic situation, in which the generation of odors can take place: for instance, bacterial reduction of sulfate (SO2 4 ) to hydrogen sulfide (HS) begins at redox potentials lower than 50 mV (Boon, 1995) and mercaptans (a general structure of R-SH, in which R = carbon chain) can also be formed (Gostelow, Parsons and Stuetz, 2001). The odor control potential of PAA dosing is discussed in the next section (see Section 3.2). 3.2. Odor control Odours in wastewater or sludge are formed as a result of several chemical and biochemical reactions but a general feature leading to the malodour formation is the development of anaerobic or septic conditions (i.e., no molecular oxygen or oxidized nitrogen species present) (Boon, 1995). One of the most common sources of odor, dihydrogen sulphide (H2S), is formed as sulfate reducing

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Fig. 3. Redox potential and odor (NH3 and H2S) concentrations in air of septic municipal wastewater sludge as a function of the PAA dose.

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Fig. 4. E. coli and Salmonella amounts of sludge samples (before centrifuging) as a function of the PAA dose. For the PAA dose 480 mg/L, the results were <10 CFU/g.

bacteria (SRB) use sulfate as an electron acceptor to form hydrogen sulfide (HS). At pH < 7, H2S becomes the dominant species and is easily volatelized (Park et al. 2014). To demonstrate the odor control with PAA in sludge conditioning, a septic sludge sample (i.e., initial redox potential  350 mV) was studied (Fig. 3). The addition of PAA (and hydrogen peroxide in the solution) prevents the formation of H2S odors by directly oxidizing HS and H2S and by supplying dissolved oxygen (i.e, increasing redox potential). When pH is between 6 and 9 (as in the present study), the oxidation of H2S and HS by hydrogen peroxide can be presented with Eq. (4) (Bonani, 1998). In the case of PAA, the exact reaction with H2S or HS has not been reported but it likely occurs via similar kind of direct oxidation or formation of hydroxyl and other radicals as a first step. A complete removal of H2S emissions was observed at 24 mg/L PAA dose coinciding with the redox potential higher than approximately 200 mV (Fig. 3), which is in agreement with earlier literature (Delgado et al., 1999).

sludge hygienization experiments, it was found that only the largest studied dose, 480 mg/L PAA, was able to reach these target values (Fig. 4). The results are in agreement with earlier studies, in which the required PAA dose for Salmonella elimination in sludge has been 250–500 mg/L (Baldry and Fraser, 1988; Fraser et al., 1984). The relatively high required PAA dose is explained by the protection of microbes by the presence of large amounts of suspended solids as summarized in (Luukkonen and Pehkonen, 2017). It is likely that the effective solubilization of organic matter and the destruction of extracellular polymeric substances, which offer protection to microbes, occurs between 50 and 100 mg/L PAA dose as indicated by a significant decrease of microbes in this dosing region.

H2 S(aq) + HS (aq) + 2H2 O2 (aq) ! zS0 (aq) + Sx Oy (aq) + aO2 (aq) + bH2 O(l)

The CO2 production and phytotoxicity of soil amended with different amounts of PAA-treated sludge was measured to evaluate (1) how much sludge could be mixed into soil without introducing excessive microbial activity or inhibition of plant growth and (2) how the PAA concentration affects microbial respiration or plant growth of soil-sludge mixtures. Conventionally, the CO2 production test is used as an indication of the compost maturity (i.e., nontoxicity and the release of nutrients) as it measures the microbial respiration, and thus, indirectly, the availability of easily biodegradable carbon sources (Itävaara et al., 2010). A CO2 production value less than 3 mg CO2-C/g VS/d (VS = volatile solids) indicates sufficiently low microbial respiration (Itävaara et al., 2010). Based on the results, sludge amounts more than 1 vol% cause clearly too high CO2 production values when using a constant PAA dose of 120 mg/L (Fig. 5A). This is due to the organic matter present in raw sludge. Moreover, the CO2 production of sludge samples appears to be independent of the applied PAA dose in the range of 0–240 mg/L (Fig. 5A). This is an expected result since PAA is not usually able to decompose organic matter present in wastewaters into CO2 but in fact increases total organic carbon (TOC) or chemical oxygen demand (COD) due to the introduction of acetic acid (Luukkonen et al., 2014; Lazarova et al., 1998; Cavallini et al., 2013). The effect of different PAA doses (0–240 mg/L) on microbial respiration of sludge samples (before centrifuging and mixing into soil) was confirmed by using another analysis method: biological oxygen demand (see supporting information, Fig. S2). In this test, sludge samples treated with 0–240 mg/L of PAA behaved identically in terms of oxygen consumption for the first approximately 100 h followed by a slight variation between samples which is, however, likely due to inhomogeneity of samples.

ð4Þ

In addition, the formation of ammonia (NH3) odors were also diminished as a result of PAA dosing. One possible explanation is the shift in the equilibrium of NH+4–NH3 (pKa = 9.23 at 25 °C) towards the direction of ammonium (NH+4) due to the slight decrease of pH (see Fig. 2A) according to Eq. (5).

NH4 þ (aq) $ NH3 (aq) + Hþ (aq)

ð5Þ

However, PAA does not react directly with ammonium (Fraser et al., 1984), which was also confirmed in the present study (see supporting information). However, it has been reported that a relatively slow reaction between aqueous ammonia and OH radicals is possible (Huang et al., 2008). PAA and hydrogen peroxide can generate OH radicals in the sludge matrix as there are transition metal catalysts present. Therefore, the observed ammonia odor abadement is likely a combination of the aforementioned two effects. 3.3. Hygienization of sludge One of the most important goals in municipal wastewater sludge stabilization is to decrease the amount of pathogenic microbes to safe levels. Conventional indicators for sludge microbial quality are E. coli and Salmonella. In Finland, for sludge to be used as a fertilizer, their levels should be 1000 colony forming unit (CFU) per g (or 100 if used in greenhouses) for E. coli and not detectable per 25 g for Salmonella (Ministry of Agriculture and Forestry of Finland, 2011). In the

3.4. Microbial respiration and phytotoxicity of sludge-soil mixtures

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Fig. 5. (A) CO2 production and (B) phytotoxicity of sludge-soil mixtures as a function of conditioned sludge amount in soils using a constant 120 mg/L PAA dose and as a function of PAA dose using a constant sludge amount of 1%.

Phytotoxicity is caused by the presence of organic or inorganic toxic compounds in soil hindering the plant growth (Itävaara et al., 2010). Typically phytotoxicity decreases as a result of sludge stabilization processes, such as aerated composting, resulting the decomposition of some of the toxic organic compounds (Itävaara et al., 2010). The phytotoxicity test results (Fig. 5B) indicated that up to approximately 3 vol% of sludge treated with 120 mg/L of PAA could be added without inhibiting plant growth. It was observed that phytotoxicity decreased slightly when the PAA dose was increased (up to 240 mg/L, keeping sludge amount constant 1 vol %). These effects were likely mainly caused by organic compounds of the sludge (the amount of potentially toxic metal(loid)s were found to be low, see supporting information, Table S1), which were increasingly oxidized as the PAA dose was increased.

3.5. Decomposition of PAA in sludge The decomposition of PAA in the sludge matrix closely followed the modified first-order kinetic rate law taking into account the initial PAA consumption (Fig. 6). The obtained rate constant (k = 0.0181 1/min) was approximately in the same range as with primary, secondary, or tertiary wastewaters (Falsanisi et al., 2006; Dell’Erba et al., 2004; Luukkonen et al., 2015). However, the observed initial decomposition (D = 33.4 mg/L) was relatively high although a value of D = 19.4 mg/L has been reported earlier for primary wastewater (Falsanisi et al., 2006). The initial consumption reflects the instantaneous reactions taking place when PAA is dosed: for instance, the oxidation of reduced species such as sulfides (see section 3.2). The values of rate constant and initial consumption increase as the suspended solids and organic matter amounts increase (Domínguez Henao et al., 2018a,b). Therefore,

Fig. 6. Decomposition of PAA (C0  100 mg/L) in the sludge matrix modelled with the modified first order kinetic model taking into account the initial PAA consumption. Dashed lines correspond to 95% confidence intervals of fitting the model to the experimental data.

the half-life of PAA is short in the studied sludge matrix with a large amount of suspended solids (5.4%): half-life was approximately 15 min in this case. 3.6. Macronutrient and potentially toxic element amounts in sludgesoil mixtures Macronutrient (N, P, K, Ca, Mg, and S) amounts in sludge-soil mixtures using different sludge amounts (1, 5, and 10 vol%) and

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Fig. 7. Nutrient amounts in sludge-soil mixtures containing different amounts of sludge as a function of the PAA dose. The dashed horizontal lines refer to the reference sample with 0% of sludge and 0 mg/L PAA.

PAA doses (0, 120, and 240 mg/L) are shown in Fig. 7. The nutrient amounts are reported as mg/L in the extraction solutions used in the test (see Section 2.3 for details) and are compared to the Finnish recommendations (Eurofins Viljavuuspalvelu Oy, 2019; The Finnish Association of Landscape Industries, 2019) in the below discussion. The Finnish nutrient level recommendations for agricultural soils have been developed based on the data collected in the course of over 50 years (Keskinen et al., 2016).

The amount of nitrogen (i.e., NH+4 and NO 3 ) correlated poorly with the added sludge amount (Fig. 7A and B) but there is a substantial increase compared to a reference sample (with 0% of sludge and 0 mg/L PAA). Moreover, the amount of NH+4 varied considerably when applying different PAA concentrations even though NH+4 does not react with PAA (Fraser et al., 1984). Ammonium (NH+4), as a readily adsorbable species, might have been partially associated with particulates rather than being present as a

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dissolved species resulting variation when dosing sludge to soil. The weight ratio of NO3-N/NH4-N was 0.1–0.6 (should be preferably greater than 1) in the present study indicating a low level of biodegradation (Itävaara et al., 2010). Some of the phytotoxicity observed in Fig. 5B could have been caused by ammonium (Itävaara et al., 2010). Nevertheless, the observed soluble nitrogen (i.e., NH+4 and NO 3 ) amounts (14–53 mg/L) are within the Finnish recommendation (15–100 mg/L) for plant substrates (The Finnish Association of Landscape Industries, 2019). Phosphorus amount increases in a consistent manner with different PAA doses as a function of sludge amount (Fig. 7C). The measured phosphorus amounts are higher than in the reference soil without sludge and PAA (dashed line in Fig. 7C). The observed values (16–35 mg/L) are largely within the recommendations for plant substrate (5–30 mg/L) (The Finnish Association of Landscape Industries, 2019) and agricultural soil (less than 20–35 mg/L depending on the soil type) (Eurofins Viljavuuspalvelu Oy, 2019). The amounts of Ca, K, Mg, and S appear to be unaffected by the increasing sludge dose (Fig. 7D–G). Their amounts are approximately similar as the reference sample without sludge and PAA, that is, the studied sludge sample appears to be a poor source for these nutrients. Their amounts are within the ‘‘good” or ‘‘high” limits (depending on the soil type) according to the Finnish recommendations (Eurofins Viljavuuspalvelu Oy, 2019). The concentration of potentially toxic elements (As, Ba, Cd, Co, Cr, Cu, Mo, Ni, Pb, Hg, Sb, V, and Zn) were determined from the sludge-soil mixtures with 1% of sludge treated with 0, 120, or 240 mg/L of PAA and from soil without sludge (see Supporting information, Table S1). The mixing of 1 vol% of sludge to soil did not increase significantly the levels of the studied metal(loid)s compared to the background (i.e., soil without sludge). Furthermore, there are no significant changes in the metal(loid) amounts when PAA dosing is increased from 0 to 240 mg/L. However, since PAA is known to destroy extracellular polymeric substances and consequently affecting organic matter solubilization and sludge disintegration (Zhang et al., 2016), the PAA treatment might affect the bioavailability of potentially toxic elements. This aspect should be evaluated in future studies. 3.7. Cost of PAA-based sludge conditioning Based on the above results, the required PAA dose for hygienization is 480 mg/L (as 100% PAA per sludge volume before thickening). The price of 12% PAA solution is 1100–1200 €/t (Luukkonen et al., 2015), and thus the sludge conditioning chemical cost would be 4.00–4.36 €/m3 of sludge before thickening or approximately 74–81 €/t of dry solids. As a comparison, the operational cost range for conventional sludge treatment methods (such as lime stabilization, composting, thermal drying, or incineration) is 200–800 €/t of dry solids including salaries, electricity, fuels, and demand for obtained product (Kalderis et al., 2010). Therefore, the PAA-based conditioning could represent a viable option in the case when a quick treatment is required, available space for sludge processing is limited, and there are agricultural entities nearby, thereby creating demand for the conditioned sludge. Moreover, PAA-conditioned sludge could replace nitrogen and phosphorus fertilizers, thereby at least partially introducing additional economic drivers for its use. 4. Conclusions Suitability of PAA for the conditioning of sludge before thickening and dewatering was studied. PAA prevented the formation of H2S and NH3 odors (at dose 25 mg/L) and was able to provide hygienization (at dose 480 mg/L). Maximum amounts of

PAA-conditioned sludge that could be mixed with soil were 3 and 1 vol% in terms of phytotoxicity and CO2 production values, respectively. The macronutrient (N, P, Ca, K, Mg, S) amounts or availability in soil were not detrimentally affected by PAA. The chemical cost of PAA treatment was 74–81 €/t of dry solids, which can be competitive against other conventional treatment options. On the basis of this study, PAA could be considered as a sludge treatment option in locations with a limited space (since no other conditioning methods might be required) and requirement for quick treatment. Furthermore, the promising odor prevention results indicate that PAA-based treatment could be feasible for wastewater treatment plants located near residential areas. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgements Mr. Teuvo Kekko and the whole staff of the former PAC-Solution Oy are acknowledged for providing this interesting research topic and co-operation during the study. The authors wish to also thank M.Sc. Ville Kaikkonen for aiding in some of the laboratory experiments. This work was supported by the Academy of Finland (grant #315103). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.wasman.2019.11.004.

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