Coupling hydrothermal carbonization with anaerobic digestion: an evaluation based on energy recovery and hydrochar utilization

This work evaluates the effect of hydrothermal carbonization (HTC) as a pretreatment and post-treatment technique to anaerobic digestion (AD) of dairy sludge. HTC's effect on AD was evaluated based on energy recovery, nutrient transformation, and hydrochar utilization. The first approach was executed by performing HTC under a range of temperatures before mesophilic AD. HTC optimal pretreatment temperature was 210 °C for 30 min residence time. HTC pretreatment significantly increased the methane yield potential by 192%, the chemical oxygen demand removal by 18%, and the sludge biodegradability during AD by 30%. On the other hand, the application of HTC after AD (post-treatment) increased the total energy production, i.e., in addition to methane, a hydrochar with a caloric value of 10.2 MJ/kg was also obtained. Moreover, HTC post-treatment improved the steam gasification performance of the AD digestate. From the fertilizer quality point of view, HTC implementation generally boosted the concentrations of macro, micro, and secondary nutrients, suggesting its suitability for use as a liquid fertilizer. Overall, the findings of the present study indicate that if bioenergy production were the main target, HTC post-treatment following AD would lead to the most promising outcomes.

Differing Phosphorus Crop Availability of Aluminium and Calcium Precipitated Dairy Processing Sludge Potential Recycled Alternatives to Mineral Phosphorus Fertiliser

The European dairy industry generates large volumes of wastewater from milk and dairy food processing. Removal of phosphorus (P) by complexing with metal (e.g., aluminium, calcium) cations in P rich sludge is a potential P source for agricultural reuse and P recycling. However, there is a significant knowledge gap concerning the plant availability of this complexed P in comparison to conventional mineral P fertiliser. The current absence of information on plant P bioavailability of dairy processing sludge (DPS) limits the ability of farmers and nutrient management advisors to incorporate it correctly into fertiliser programmes. The present study examined the most common types of dairy sludge—(1) aluminium-precipitated sludge (“Al-DPS”) and (2) calcium-precipitated lime-stabilised sludge (“Ca-DPS”) at field scale to assess P availability in grassland versus mineral P fertiliser over a growing season. The experimental design was a randomised complete block with five replications. Crop yield and P uptake were assessed for 4 harvests. The initial soil test P was at a low level and the experimental treatments were super phosphate at 15, 30, 40, 50 and 60 kg P ha−1, two dairy sludge applied at 40 kg P ha−1 (comparison was made with mineral P at same application rate) and a zero P control applied in a single application at the beginning of the growing season. Results showed a significant positive slope in the relationship between P uptake response and mineral P application rate indicating the suitability of the experimental site for P availability assessment. The P bioavailability of Al- and Ca-DPS varied greatly between treatments. The P fertiliser replacement value based on the 1st harvest was 50 and 16% increased to 109 and 31% cumulatively over the four harvests for Al- and Ca-DPS, respectively. The Al concentration in Al-DPS did not limit P bioavailability, but low P bioavailability from Ca-DPS can be associated with its high Ca content that can lead to formation of low soluble Ca-P compounds at alkaline pH conditions with a high Ca/P ratio. These findings show that P availability from dairy sludge can be quite different depending on treatment process. Consequently, it is critical to have P availability information as well as total P content available to ensure the application rate meets crop requirements without creating environmental risk by over application.

Precision Injection of Dairy Sludge on Crop Yield and N and P Uptake in Juvenile and Mature No-Till Silage Corn

Starter mineral fertilizer is used by famers to provide phosphorus (P) and nitrogen (N) to emerging corn (Zea mays) plants. Recent studies have shown that dairy slurry can replace mineral fertilizer provided it is precisely positioned close to the corn rows. This 5-year study examined the multi-year effect of precision injected sludge, the thick fraction separated from dairy slurry, on growth and nutrient uptake at the 6-leaf stage and final harvest of no-till corn. The sludge was first injected 15 cm deep and the corn planted < 10 cm from the sludge furrow at least 3 days later. Sludge provided sufficient P for both early growth and full season growth. At final harvest with equivalent total N and P rates (32P 250N treatments), fertilizer and sludge had similar maximum yields (17.9 and 17.4 t ha−1, respectively) and P uptake (26 and 25 kg ha−1, respectively) but fertilizer had higher N uptake than sludge (200 and 162 kg ha−1). N uptake and recovery N use efficiency was greater for sludge than fertilizer based on equivalent min.-N which suggests crop benefits in the sludge other than min-N and P. The study shows that precisely injected dairy sludge can obviate the need for starter mineral fertilizer, and this may help to alleviate P surpluses on dairy farms. This practice also provides a use on dairy farms for the separated solids fraction remaining after the thin fraction is decanted and applied as the primary N source to grass.

Improving biogas production performance of dairy activated sludge via ultrasound disruption prior to microwave disintegration

In this study, ultrasound disruption was employed to enhance the efficiency of microwave disintegration of dairy sludge. Results revealed that ultrasound specific energy input of 1,500 kJ/kg TS was found to be optimum with limited cell lysis at the end of the disruption phase. Biodegradability study suggested an enhancement in suspended solids reduction (16%) and biogas production (180 mL/gVS) in floc disrupted (deflocculated) samples when compared to sole microwave pretreatment (8.3% and 140 mL/gVS, respectively). Energy assessment to attain the 15% optimum solubilization revealed a positive net production of 26 kWh per kg sludge in deflocculated samples compared to 18 kWh in flocculated (sole microwave) samples. Thus, ultrasound disruption prior to microwave disintegration of dairy sludge was considered to be a feasible pretreatment technique.

An evaluation of lysozyme enzyme and thermal pretreatments on dairy sludge digestion and gas production

Anaerobic digestion is one of the common methods of managing and stabilizing sludge. However, due to the limitations of the biological sludge hydrolysis stage, anaerobic decomposition is slow and requires a long time. This study evaluated the effects of thermal (80 °C) (TH-PRE) and a combination of thermal with the lysozyme enzyme (LTH-PRE) pretreatments on the enhancement of anaerobic activated sludge digestion. Response surface methodology was implemented to optimize enzyme pretreatment conditions (enzyme and mixed liquid suspended solids concentration). The results showed that both pretreatment methods increase soluble chemical oxygen demand (COD) and reduces total and volatile suspended solids (VSS), and phosphate concentration. The COD removal rate in LTH-PRE and TH-PRE was 95% and 81%, respectively. The value of VSS reduction in LTH-PRE and TH-PRE was 41% and 31%, more than the control operation, respectively. The biogas production in LTH-PRE and in TH-PRE also increased by 124% and 96%, respectively.