MOKSLAS – LIETUVOS ATEITIS SCIENCE – FUTURE OF LITHUANIA

Iron salts are used at wastewater treatment plants (WWTPs) for several reasons: for removing chemical phosphorus, preventing from struvite formation and reducing the content of hydrogen sulfide (H2S) in biogas. Anaerobic digestion is a common scheme for sludge treatment due to producing biogas that could be used as biofuel. Laboratory analysis has been carried out using anaerobic digestion model W8 (Armfield Ltd, UK) to investigate any possible effect of adding FeCl3 on the anaerobic digestion of primary sludge (PS) and waste activated sludge (WAS) mixture as well as on releasing phosphates to digested sludge liquor. The obtained results showed that FeCl3 negatively impacted the anaerobic digestion process by reducing the volume of produced biogas. Fe-dosed sludge (max) produced 30% less biogas. Biogas production from un-dosed and Fe-dosed sludge (min) was similar to the average of 1.20 L/gVSfed. Biogas composition was not measured during the conducted experiments. Phosphorus content in sludge liquor increased at an average of 38% when digesting sludge without ferric chloride dosing. On the contrary, phosphate content in sludge liquor from digested Fe-dosed sludge decreased by approx. 80%.


Introduction
Anaerobic digestion is a common scheme for sludge treatment because of producing biogas the content of which includes methane (CH 4 ) (55-65%), CO 2 (30-35%), small amounts of other O 2, H 2 S, etc. and is an environmentally friendly fuel. However, without further treatment, it can only be used at the place of production (Appels et al. 2008).
The anaerobic digestion of sludge leads to the formation of struvite because ammonia, phosphate and magnesium are solubilised by the digestion process (Mamais et al. 1994). One of the existing methods of avoiding the formation of struvite is adding FeCl 3 salts that could affect the anaerobic digestion process.
Iron salts could be used at wastewater treatment plants (WWTPs) for several reasons: for removing chemical phosphorus, preventing from struvite formation and reducing the content of hydrogen sulfide (H 2 S) in biogas (Mamais et al. 1994).
The effect of adding iron salts on anaerobic digestion was studied by many researchers (Smith, Carliell-Marquet 2008, 2009Novak et al. 2007) the majority of which reported a negative effect of dosing iron salts on a daily production of biogas comparing to un-dosed sludge (see Table 1).
The present study was undertaken to discover any possible effect of iron addition on the anaerobic digestion of the primary-waste activated sludge mixture and phosphorus release during the digestion of the sludge mixture.

Materials and Methods
Primary and waste activated sludge was collected from Vilnius waste water plant (Lithuania). Primary sludge (PS) was taken from the primary sludge pumping station before thickening and waste activated sludge (WAS) was collected from the distribution chamber of the secondary sedimentation tank. Fresh sludge used to be taken from the plant every 5 th day to avoid changes in sludge characterization and then stored in a refrigerator at a temperature of +4°C. The proportion of (volume basis) primary and waste activated sludge was 1:2. The average characteristics of the feed of the primary-waste activated sludge mixture are shown in Table 2. The characterization of PS could vary depending on the presence of solids in wastewater, the types of units, etc. (Smith, Carliell-Marquet 2008, 2009).
The feed was screened with a sieve to avoid large particles and was not thickened due to the narrow tubes (4 mm) of peristaltic pumps. The total solids (TS) concentration in the sludge mixture was 1.6-2.4%. Regular analysis was performed to determine the characteristics and consistency of the feed material.
This model consists of two separate high-rate digesters -4.8 L each. Reactors were continually mixed using internal mechanical stirrer. Flow rates to the vessels are set and controlled by calibrated peristaltic pumps. The temperature of each reactor was controlled by an electric heating mat wrapped around the external wall. Gas off-take from each reactor was taken into a volumetrically calibrated collector vessel (3300 ml) operating by water displacement. Each reactor was inoculated with anaerobic inoculum obtained from the previous experiment with the anaerobic digestion of the primary-waste activated sludge mixture (Ofverstrom et al. 2010).
The sludge mixture was pumped daily during 6 hours through the system using peristaltic pumps. The amount of feed sludge was 0.24 L/d with volatile solids loading (VS load ) from 0.67 to 0.88 gVS/l/d with an average of 0.76gVS/d. Sludge was not thickened due to a small di-ameter of flexible connection pipes. Anaerobic digestion was made under mesophilic conditions at a temperature of 35°C with a solid retention time (SRT) of 20 days.
Laboratory analyses were made in April-June 2010. The total amount of experiments were three -the anaerobic digestion of un-dosed and Fe-dosed sludge mixtures. The doses of Fe were selected according to the Gothenborg (Sweden) WWTP average amounts of Fe in sludge in 2008. It was obtained that the amount of Fe was 52-81 mgFe/ gTS. Minimum and maximum doses selected were 50 and 100 mgFe/gTS respectively.
The average concentration of TS in the sludge mixture was 16.8gTS/L. Fe salts selected for the conducted experiment were FeCl 3 ·6H 2 O and were dosed daily into the feed sludge mixture (See Table 3). The final concentration of Fe in the anaerobic digester was 0.84gFe/L for min and 1.68gFe/L for max doses respectively.
Process performance was routinely monitored by measuring pH, influent and effluent TS and VS, alkalinity (ALK), volatile fatty acids (VFA), biogas production and phosphate concentration in sludge liqour. The composition of biogas produced was not measured during the experiment. All analyses were performed according to Standard Methods (APHA 2000).

Results and Discussion
The examination of data on digested sludge mixture ALK, VFA and pH showed a reduction in alkalinity in digesters receiving sludge with a max dose of Fe (see Table 4). During digestion, Fe-dosed sludge with max dose pH was only 6.32-6.37, which is possibly due to lower alkalinity (only 250-450 mg/L) and probably because of FeCO 3(s) precipitation during the first days of the experiment (Mamais et al. 1994 (1) Within the first days of adding Fe salt to feed the sludge mixture, biogas production decreased by about 20-25% and 50% for Fe-dosed sludge with minimum and maximum doses (data is not showed). Biogas production was evaluated from the 10 th day of the experiment (after process stabilization). The average biogas production from the un-dosed primary-waste activated sludge mixture was 1204 mL/gVS fed , depending on the VS load of the fed sludge mixture which was similar to average biogas production from Fe-dosed sludge (min dose) -1212 mL/gVS fed . Biogas production from Fe-dosed sludge (max) was only from 500 to 1029 mL/gVS fed with an average of 832 mL/ gVS fed which were ~30% lower than from both un-dosed and Fe-dosed (min) sludge (See Table 4).
Daily biogas production (ml/gVS fed ) from the undosed and Fe-dosed sludge mixture as 2-days running averages is shown in Fig. 2.
Biogas production from un-dosed and Fe-dosed sludge with a minimum dose was decreasing during the 10 th -16 th days of the experiment probably due to feeding the stored sludge mixture.
A fresh sludge mixture was dosed from the 16 th day of the experiment which affected biogas production: Fe-dosed with a minimum dose became higher directly after feeding of fresh sludge. Biogas production from un-dosed and Fedosed sludge with a maximum dose was changing from the 18 th day and was slower than that for Fe-dosed sludge with a minimum dose. Iron is a necessary element for the growth of microorganisms as it forms an important component of a number of enzymes involved in the metabolic processes of bacteria. Iron was as a trace metal for Fe-dosed sludge with a minimum dos, was overdosed for Fe-dosed sludge with a maximum dose and was toxic for methanogens.
The average phosphate concentration in fed sludge mixture liquor was 42.7 mg/L. The average phosphate content in digested sludge liquor from un-dosed sludge increased by 38% due to the solubilization of phosphate during anaerobic digestion and varied from 42.8 to 68.4 with an average of 56.0 mg/L. The pH of un-dosed digested sludge was from 6.80 to 7.0 that was low for the precipitation of struvite (NH 4 MgPO 4 ). Struvite precipitates at pH 8.5 at higher (Mamais et al. 1994).
The effect of the pH of the digested sludge mixture on the concentration of phosphates in sludge liquor is showed in Fig. 3. Strong power correlation (R 2 = 0.869) was found between pH and phosphate concentration. During the digestion of Fe-dosed sludge, pH was lower than 6.7 and an aver-  age concentration of phosphates in digested sludge liqour was 9.2 for Fe-dosed sludge with a maximum dose and 8.6 for Fe-dosed sludge with a minimum dose. Mamais et al. 1994 suggested that when iron dose is increased, soluble PO 4 3decreases if Fe 3 (PO 4 ) 2(s) precipitates. During the experiment, phosphate content in digested Fe-dosed sludge liquor decreased by 80% for both sludges compared to the content of phosphates in fed sludge liquor.

Conclusions
The effects of adding iron salt on the digested primarywaste activated sludge mixture were investigated through experimental studies making the following conclusions: 1. The obtained results showed that the anaerobic digestion of Fe-dosed sludge (min) with the Fe concentration of 0.84 gFe/L resulted similar biogas production as from un-dosed sludge mixture. The average biogas production was 1.20 L/gVS fed . 2. Fe-dosed sludge (max) produced 30% less biogas in comparison with biogas production from un-dosed sludge. 3. The average phosphate content in digested sludge liquor from un-dosed sludge increased by 38% due to the solubilization of phosphate during anaerobic digestion. 4. Phosphate content in digested Fe-dosed sludge liquor decreased by 80% for both Fe-dosed sludges compared to phosporus content in fed sludge liquor. 5. It is necessary not to overdose Fe salt during sludge digestion. Higher Fe salt doses could negatively affect biogas production. 6. For a better undestanding of Fe effect on the anaerobic digestion process of the primary-waste activated sludge mixture, biogas composition analysis is required.