Digested Sludge Liquors Research Articles

Complex Agent for Phosphate Sequestration from Digested Sludge Liquor: Performances and Economic Cost Analysis

Phosphorus (P) management in the “water-energy-resource-nexus” in wastewater treatment plants (WWTPs) remains a longstanding challenge. P adsorption from the P-enriched digested sludge liquor (DSL) is a comparatively more practical and economically viable approach for P recovery in WWTPs. However, high concentrations of impurities in DSL might pose a negative and interferential effect on P adsorption, hindering the application of sorbents or precipitation methods. Given such a situation, highly efficient and cost-effective sorbent towards P reclamation from DSL is highly needed. Therefore, this study aims to develop a novel complex agent containing aluminum coagulant and superparamagnetic nano-sorbent (SNS) that can be used in magnetic seeding coagulation for P recovery. The complex agents with different PACl: SNS ratios showed varied turbidity removal rates and P recovery efficiencies and the optimal ratio was 15 mg PACl: 15 g SNS. PAC and SNS showed significant interaction because PAC could enhance P adsorption by shielding the interferential effect of colloidal impurities. In addition, the complex is highly regenerative, with turbidity and P removal rate stably maintained at 70–80% after 10 adsorption/regeneration cycles. The cost–benefit analysis of the dosing complex agent showed a dosing cost of 0.154 EUR/m3, admittedly much higher than the conventional magnetic seeding coagulation, which could probably be covered by the profit if the expensive and rare P product is reclaimed. This work indicated that the complex agent was superior due to its high adsorption capacity, easy separation, and repeated dosing, and therefore had the potential for P recovery from DSL.

Influence on Desulfurization Efficiency and Interactions of Fe/S and pH During H2S in situ Depression of High Solid Anaerobic Digestion

To evaluate the influence of Fe/S ratio and pH on sulfide removal efficiency and interactions between Fe/S and pH, anaerobic hydrogen sulfide in situ depression tests and digested sludge liquor sulfide removal tests were carried out by using dewatering sludge from a wastewater treatment plant (WWTP). Results showed that the concentration of hydrogen sulfide in biogas from the thermal pretreatment following anaerobic digestion process could be reduced from 170.4×10-6 to 14.09×10-6 at Fe/S=7.75, which means the biogas desulfurization treatment is not required. Under the condition of pH 7.00-7.50 and Fe/S 1-11, pH is the main influencing factor for sulfide removal. Improving the pH of anaerobic digestion is beneficial in reducing the dosage of Fe(Ⅲ). An Fe/S ratio of 7.0 is the minimum to meet the biogas hydrogen sulfide emission standards during high solid sludge anaerobic digestion. The concentration of hydrogen sulfide was not up to standards if pH was below 7.30.

Advances in wastewater nitrogen removal by biological processes: state of the art review

The paper summarizes the state-of-the-art of the most recent advances in biological nitrogen removal, including process design criteria and technological innovations. With reference to the Modified Ludzck Ettinger (MLE) process (pre-denitrification and nitrification in the activated sludge process), the most common nitrogen removal process used nowadays, a new design equation for the denitrification reactor based on specific denitrification rate (SDNR) has been proposed. In addition, factors influencing SDNR (DO in the anoxic reactor; hydrodynamic behavior) are analyzed, and technological solutions are proposed. Concerning technological advances, the paper presents a summary of various “deammonification” processes, better known by their patent names like ANAMMOX®, DEMON®, CANON®, ANITA® and others. These processes have already found applications in the treatment of high-strength wastewater such as digested sludge liquor and landfill leachate. Among other emerging denitrification technologies, consideration is given to the Membrane Biofilm Reactors (MBfRs) that can be operated both in oxidation and reduction mode.

Microbial community analysis of a full-scale DEMON bioreactor.

Full-scale applications of autotrophic nitrogen removal technologies for the treatment of digested sludge liquor have proliferated during the last decade. Among these technologies, the aerobic/anoxic deammonification process (DEMON) is one of the major applied processes. This technology achieves nitrogen removal from wastewater through anammox metabolism inside a single bioreactor due to alternating cycles of aeration. To date, microbial community composition of full-scale DEMON bioreactors have never been reported. In this study, bacterial community structure of a full-scale DEMON bioreactor located at the Apeldoorn wastewater treatment plant was analyzed using pyrosequencing. This technique provided a higher-resolution study of the bacterial assemblage of the system compared to other techniques used in lab-scale DEMON bioreactors. Results showed that the DEMON bioreactor was a complex ecosystem where ammonium oxidizing bacteria, anammox bacteria and many other bacterial phylotypes coexist. The potential ecological role of all phylotypes found was discussed. Thus, metagenomic analysis through pyrosequencing offered new perspectives over the functioning of the DEMON bioreactor by exhaustive identification of microorganisms, which play a key role in the performance of bioreactors. In this way, pyrosequencing has been proven as a helpful tool for the in-depth investigation of the functioning of bioreactors at microbiological scale.

THE EFFECT OF IRON SALT ON ANAEROBIC DIGESTION AND PHOSPHATE RELEASE TO SLUDGE LIQUOR / GELEŽIES DRUSKOS ĮTAKA ANAEROBINIO DUMBLO PŪDYMO PROCESUI IR FOSFATŲ IŠSISKYRIMUI Į DUMBLO VANDENĮ

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%. Santrauka Nuotekų valymo įrenginiuose geležies druskos naudojamos cheminiam fosforui šalinti, sieros vandeniliui biodujose mažinti ir struvito nuogulų formavimosi prevencijai. Tyrimai atlikti laboratorinėmis sąlygomis naudojant anaerobinio pūdymo modelį W8 (Armfield Ltd., Didžioji Britanija) ir pūdant pirminio perteklinio dumblo mišinį, į kurį buvo dedama geležies druskos, siekiant nustatyti geležies druskos naudojimo efektą anarobinio pūdymo procesui ir fosfatų išsiskyrimui į dumblo vandenį. Rezultatai parodė, kad pūdant dumblą be geležies druskos ir dedant geležies druskos minimalią dozę, susidarė vidutiniškai vienodi kiekiai biodujų (mL/gBSMtiekiam.), bet į pūdomą dumblą dedant maksimalią geležies dozę, biodujų išeiga vidutiniškai sumažėjo 30 %. Į pūdomą dumblą nededant geležies chlorido, fosfatų koncentracija dumblo vandenyje vidutiniškai padidėjo 38 %. Ir priešingai, geležies chlorido dozės fosfatų koncentraciją pūdyto dumblo vandenyje sumažino 80 %.

Ammonium removal from digested sludge liquors using ion exchange

Liquors arising from the dewatering of digested sludge typically contain ammonium levels in the range 200–700 mg l −1 NH 4 +-N. These liquors are frequently recycled to the head of the wastewater treatment works (WwTW) untreated and can constitute >25% of the total nitrogen load entering the works at inlet. This paper investigates the use of a clay-based material, MesoLite, as an ion exchange medium for ammonium removal from recycle streams. Pilot-scale studies performed at Didcot WwTW, part of the Thames Water wastewater treatment network, indicate that MesoLite is highly selective for the ammonium ion. Results show that >95% of ammonium was removed from belt press liquors with an initial ammonium nitrogen concentration >600 mg l −1, with an overall ion exchange capacity >51 g NH 4 +-N kg −1 medium and this resulted in an operating capacity in the range 27–36 g NH 4 +-N kg −1.

Solutions to a combined problem of excessive hydrogen sulfide in biogas and struvite scaling

The Woodman Point Wastewater Treatment Plant (WWTP) in Western Australia has experienced two separate problems causing avoidable maintenance costs: the build-up of massive struvite (MgNH4PO4. 6H2O) scaling downstream of the anaerobic digester and the formation of hydrogen sulfide (H2S) levels in the digester gas to levels that compromised gas engine operation and caused high operating costs on the gas scrubber. As both problems hang together with a chemical imbalance in the anaerobic digester, we decided to investigate whether both problems could be (feasibly and economically) addressed by a common solution (such as dosing of iron solutions to precipitate both sulfide and phosphate), or by using separate approaches. Laboratory results showed that, the hydrogen sulfide emission in digesters could be effectively and economically controlled by the addition of iron dosing. Slightly higher than the theoretical value of 1.5 mol of FeCl3 was required to precipitate 1 mol of dissolved sulfide inside the digester. Due to the high concentration of PO4(3-) in the digested sludge liquor, significantly higher iron is required for struvite precipitation. Iron dosing did not appear an economic solution for struvite control via iron phosphate formation. By taking advantage of the natural tendency of struvite formation in the digester liquid, it is possible to reduce the risk of struvite precipitation in and around the sludge-dewatering centrifuge by increasing the pH to precipitate struvite out before passing through the centrifuge. However, as the Mg2+/PO4(3-) molar ratio in digested sludge was low, by increasing the pH alone (using NaOH) the precipitation of PO4(3-) was limited by the amount of cations (Ca2+ and Mg2+) available in the sludge. Although this would reduce struvite precipitation in the centrifuge, it could not significantly reduce PO4(3-) recycling back to the plant. For long-term operation, maximum PO4(3-) reduction should be the ultimate aim to minimise PO4(3-) accumulation in the plant. Magnesium hydroxide liquid (MHL) was found to be the most cost-effective chemical to achieve this goal. It enhanced struvite precipitation from both, digested sludge and centrate to the point where more than 95% PO4(3-) reduction in the digested sludge was achieved.

Chemical Control of Struvite Precipitation

The control of struvite precipitation was investigated using a range of chemical inhibitors and chelating agents. Initial experiments investigated the mass of struvite that precipitated on jar tester impellers used to mix anaerobically digested sludge liquors. Mixing the liquors resulted in degassing of CO2 increasing the pH from below 7 to above 8. It was found that the greater the liquors’ supersaturation ratio (Ω) the greater the mass of struvite formed on the impellers. Liquors with a Ω=1.7 and 5.3 produced mean final masses of 73.8 and 136g/m2, respectively, after 40 h of mixing. Experiments using the same impellers were performed with a range of scale inhibitors and chelating agents. The scale inhibitors did not prevent struvite from forming, with only one product, Polystabilis, showing a reduction in the mass of scale formed on the impellers. Of the chelating agents tested, ethylenediaminetetraacetic acid (EDTA) was found to be the most effective. Further experiments showed that an EDTA: struvite ratio of 2:1 dissolved >90% of struvite back into solution even at pH values greater than 7.5.

The Use of High-Rate Nitrification for the Pretreatment of Ammoniacal Digested Sludge Liquors

Anaerobic sludge digestion reduces most of the organic nitrogen in raw sludge to ammonia. Where the digested sludge is dewatered, the ammonia in the liquors can represent up to 25% of the total ammonia load on the receiving wastewater-treatment plant. This can cause problems with compliance and also limit the throughput of the digestion plant. The paper describes how Anglian Water addressed this problem at Cliff Quay wastewater-treatment plant, Ipswich by installing the Amtreat high-rate nitrifying process. The reasons for the necessity to reduce the ammonia load to the plant are presented and the rationale behind the selection of this solution is outlined. The Amtreat plant is described and its performance is evaluated. The Cliff Quay site is the first application of this process to a municipal wastewater-treatment plant in the UK, although there are several installations operating with industrial effluents.

Successful sidestream nitrification of digested sludge liquors

When digested sludge is dewatered for recycling to agricultural land or incinerated the centrate/filtrate liquors present considerable ammonia loads on the receiving sewage treatment plants which can compromise effluent standards. As an alternative to mainstream treatment at the sewage works sidestream treatments have been evaluated. At two incinerator sites sidestream ASPs processes have been chosen, and the outcome of modelling, laboratory studies and pilot plant operation are discussed, along with the design of the two plants and full scale performance.

Successful sidestream nitrification of digested sludge liquors

When digested sludge is dewatered for recycling to agricultural land or incinerated the centrate/filtrate liquors present considerable ammonia loads on the receiving sewage treatment plants which can compromise effluent standards. As an alternative to mainstream treatment at the sewage works sidestream treatments have been evaluated. At two incinerator sites sidestream ASPs processes have been chosen, and the outcome of modelling, laboratory studies and pilot plant operation are discussed, along with the design of the two plants and full scale performance.

A novel high rate method for the nitrification of sludge liquors containing high levels of ammonia

A high rate nitrification system is proposed for the nitrification of digested sludge liquors with N-NH3 levels of between 500 and 1,000 mg/l. Bench and pilot scale studies have established that loading rates of between 1.0 and 2.0 kg N-NH3/m3 of reactor volume/day are achievable by maintaining a constant 30° C reactor temperature. The effluent standard was set at 40 mg/l of N-NH3. Loading rates in excess of 1.0 kg N-NH3/m3 of reactor volume/day required supplementing conventional fine bubble diffusion with pure oxygen. pH and alkalinity control were utilised to maintain the pH in the range 7.2 – 7.6 and total alkalinity in excess of 250 mg/l (as CaCO3) by the use of a feed mixture of NaOH and NaCO3 (20 and 40 g/l respectively). The nitrifying population was fixed and stabilised within the reactor by using reticulated foam blocks (25 mm × 25 mm s× 25 mm) occupying approximately 30% of the reactor volume. Settlement was achieved by conventional clarification with a nominal up flow velocity of 1.0 m/hour.