Improving the nitrogen retention and fertilizer value of thermally dried digestate solids by addition of strong acid, alum or zeolite

Research output: Book/ReportPh.D. thesisResearch

  • Jingna Liu
Thermal drying is an increasingly common post-treatment for digestate solids, but results in nitrogen (N) losses via ammonia (NH3) volatilization. Acidification with strong acids prior to drying may retain ammonium nitrogen (NH4+-N) in the solids. As alternative approaches, natural zeolites can capture free ammonia and provide adsorption sites with exchangeable cations for ammonium ions; while aluminium sulphate (Al2(SO4)3·nH2O) (commonly referred to as alum), could hydrolyse to the aluminium hydroxide precipitate and a dilute sulfuric acid solution, resulting in a lower pH of the mixture and thus conserving nitrogen in NH4+ form to avoid volatilization losses.
The present study focused on evaluating the efficiency of these three additives on nitrogen retention during thermal drying and the effect of acidified-dried digestate solids on soil nutrient (in particular nitrogen and phosphorus) dynamics and plant growth.
The first study investigated whether the zeolite addition increased ammonium nitrogen retention during thermal drying of two digestate solids (manure based, MDS; sewage sludge based, SDS) and whether any synergistic effects of combining acidification and zeolite addition occurred. Operating conditions included four pH levels (non-acidified control, adjusted to 8.0, 7.5, 6.5, respectively, with concentrated sulfuric acid, H2SO4), four zeolite addition-rates (0%, 1%, 5% and 10%), a fixed drying temperature (130oC) and fixed air ventilation rate (headspace exchange rate of 286 times hour-1). Zeolite addition significantly increased NH4+-N retention from 18.0% of initial NH4+-N in the non-acidified control up to a maximum of 57.4% in MDS, and from 76.6% to 94.5%, respectively, in SDS. Moreover, combing sulfuric acid and zeolite addition did not further improve NH4+-N retention during drying. Nevertheless, zeolite has the potential to be a safe and easy-to-handle alternative to concentrated sulfuric acid.
The second study evaluated the effects of alum addition on NH4+-N retention and phosphorus (P) solubility in dried digestate solids in comparison to the addition of concentrated sulfuric acid. MDS and SDS were chosen to conduct a drying experiment at four pH levels (original pH, 8.0, 7.5 and 6.5) and using two acidifying agents (alum, concentrated H2SO4). Alum addition significantly reduced ammonia loss, and the final NH4+-N content increased from 18% (of initial NH4+-N) in the non-acidified control up to 234% (representing an increase in NH4+-N from initial) in dried MDS, and from 76.6% to 121%, respectively, in SDS, which was higher than with the addition of concentrated H2SO4. Moreover, alum considerably lowered the water extractable phosphorus (WEP) in raw and dried SDS by 37-83% and 48-72%, respectively, compared with the non-treated control. In contrast, concentrated H2SO4 notably increased WEP in raw and dried MDS by 18-103% and 29-225%, respectively. The comparison between the two acidifying agents indicated that alum had the potential to be an efficient and more easy-handling alternative to concentrated sulfuric acid, resulting in higher NH4+-N retention and lower P solubility.
In the third study, treated digestate solids produced in the first and second study (raw, H2SO4-treated raw, alum-treated raw, dried, H2SO4-treated dried and alum-treated dried solids, respectively) were chosen for the evaluation of their N and P fertilizer value in a plant pot experiment with maize. For MDS, drying significantly lowered the nitrogen fertilizer replacement value (N-FRV) after soil application from 42% in the untreated to 12% in the dried material, reducing maize biomass by 34% and N uptake by 54%, compared to the untreated control. Acidification prior to drying improved the maize N uptake by 60%, resulting in more than a doubling of the N-FRV to 28%, irrespective of the acidifying agent used. Moreover, drying considerably increased maize P uptake by 25% compared to raw MDS, with dried MDS being an effective P fertilizer (P-FRV of 82%). Acidification using concentrated H2SO4 prior to drying had no additional benefit for maize P uptake, while using alum neutralized the positive effect of drying, resulting in 25% less P uptake, which was comparable to the uptake from raw MDS. For SDS, no influence of drying or acidification on maize biomass or N and P uptake was observed, but dried SDS showed a relatively high N mineralization potential after soil application, as shown in an accompanying soil incubation study. N-FRV and P-FRV of SDS remained at around 33% and 26%, respectively. The H2SO4-acidified dried MDS could therefore be used as a starter fertilizer with moderate N and high P availability, capable of fulfilling the nutrient demands of maize at its early growth stages. Alum treatment could be an alternative option for dried MDS to maintain N fertilizer value combined with a reduced P solubility preventing excess P runoff. However, the dried SDS was much less able to substitute mineral starter fertilizer N or P, and should only be applied to maintain soil organic N levels and P availability.
In conclusion, the thesis work proved that zeolite and alum are two alternative options to concentrated H2SO4 for increasing the nitrogen fertilizer value of dried digestate solids (DS). Furthermore, the soil incubation and pot trial contributed to a better understanding of the possible agricultural application areas for acidified-dried digestate solids. Nevertheless, a systematic evaluation of the influence of drying on plant P availability using a more extensive range of digestate solids is required to generalize the findings of the present study where drying significantly increased maize P uptake of MDS. Moreover, conducting drying and acidification at a larger scale (pilot scale or industrial scale) is necessary for further feasibility evaluation. The investigation of the long-term agricultural performance of acidified dried digestate solids and their influences on soil quality is also essential to complete their assessment.
Original languageEnglish
PublisherDepartment of Plant and Environmental Sciences, Faculty of Science, University of Copenhagen
Publication statusPublished - 2019

ID: 237510731