Landfill Leachate Biology: Choosing Between SBR, MBR and UASB Trains

A landfill that has been open for three years and one that has been open for fifteen hand over completely different liquors to the same plant. Young leachate carries volatile fatty acids that degrade quickly; mature leachate carries humic and fulvic fractions that resist biology almost entirely. Treating both with one fixed flowsheet is the most common reason leachate plants underperform. What follows is a practical map of the biological options, the measurements that separate them, and the point at which biology has to hand over to chemical oxidation.

Why one process never holds the line on its own

Biological degradation dominates leachate treatment for three unglamorous reasons: it removes a broad spread of pollutants, it is cheap to run, and it generates no chemical sludge that has to be dewatered and landfilled. The difficulty is that the two families of organisms want opposite conditions. Aerobic tanks need oxygen and short retention. Anaerobic reactors need long hydraulic retention and large volumes, but produce little sludge, tolerate heavy organic loads and cost less to build and operate. Neither family, working alone, brings mature leachate to a discharge standard.

Aerobic routes: SBR and MBR under real load

The sequencing batch reactor, or SBR, is a conventional activated sludge plant reorganised into time rather than space: fill, react, settle and decant all happen in one tank. The arrangement gives a simple structure, high sludge activity, fast reaction and clean solid-liquid separation, with little risk of bulking. Spagni and co-workers ran a real-time controlled SBR on leachate and reported average removal of 98% for ammonia nitrogen and 95% for total nitrogen, but chemical oxygen demand (CODcr) removal stalled at 20%-30%. That single gap explains why SBR is almost never the last stage. Research effort has since moved towards bioaugmented SBR and towards SBR combined with other processes; Sun Tiegang's team paired bioaugmentation with an SBR-AO train in a field pilot and found the augmented system clearly ahead of the conventional one.

Membrane bioreactors, MBR for short, place a membrane separation unit inside the biological system and are among the most actively studied options in China. Wang Jinhui et al. used a submerged polypropylene MBR and recorded strong COD and ammonia nitrogen removal, with suspended solids in the effluent close to non-detectable; the same trial showed membrane flux falling fast and staying low. Ouyang Ke et al. worked with a submerged MBR and tracked COD, ammonia nitrogen (NH4+-N) and total nitrogen (TN) together. Leachate COD came down to 650-1500 mg/L. Fouling followed a two-stage pattern, and combined cleaning with alkali and an oxidant restored performance and brought the filtration pressure back down.

Anaerobic pretreatment with UASB reactors

Anaerobic treatment does its work on strength rather than on polish. It suits leachate with poor biodegradability and high organic concentration, and because the upflow anaerobic sludge blanket (UASB) reactor holds a dense granular blanket it separates solids from liquid unusually well. That is why UASB is the most widely applied anaerobic process in leachate duty today. Wang Wei et al. examined an improved externally circulating UASB and found a high biogas yield under heavy loading when the reactor took high-strength leachate. COD removal looked healthy, yet the effluent still could not be trusted to meet limits and needed advanced treatment downstream. A UASB is a good first stage and a poor last one.

Combined trains and where the ammonia line sits

Combining the two families is standard practice, and the reason is arithmetic rather than fashion. Anaerobic stages are slow but cheap per kilogram of COD destroyed; aerobic stages are fast but cannot carry the load alone. Jin Yongxiang et al. ran a composite A/O system on mature leachate and brought effluent ammonia nitrogen (NH4+-N) below the 20 mg/L line required by the Standard for Pollution Control on the Landfill Site of Municipal Solid Waste. Wu Lina et al. pushed further with a UASB1 + A/O + UASB2 arrangement in which the downstream UASB stage reuses the carbon source left in the upstream effluent, reaching 97% removal of NH4+-N and 95% for TN.

Aged leachate often needs help that process configuration alone cannot supply. Dosing a composite microbial agent is one route: strains are acclimatised and cultured by bioengineering methods before being added. Li Hong et al. dosed a BM composite agent and confirmed a clear advantage over an undosed control, both in performance and in the mechanism behind it. Designers have also borrowed from soil biology. The JS-BC process, derived from conventional activated sludge and the rotating biological contactor, proliferates and activates soil bacteria (Bacillus) so that BOD, COD, NH4+-N, TN and TP are removed together while the odour the system generates is broken down at the same time.

Oxidation as the final stage, not a substitute

Advanced oxidation processes (AOPs) generate hydroxyl radicals under electricity, light, catalysts or oxidants; those radicals attack refractory organics and can mineralise them all the way to carbon dioxide and water. Fenton chemistry - ferrous iron plus hydrogen peroxide (H2O2) - is the cheapest entry point. Chen Di et al. applied it to circulating semi-aerobic leachate and, at optimum conditions, removed 84.77% of CODcr and 60% of colour. The classic drawbacks remain: catalyst recovery is awkward, the reaction wants low pH, iron-bearing sludge is voluminous, residual Fe2+ risks secondary pollution, and reagent consumption is high. Sono-Fenton, photo-Fenton and electro-Fenton exist to soften those limits; Mohajeri et al. ran electro-Fenton on leachate and reached 94.07% COD removal with 95.83% for colour.

Ozone carries a redox potential second only to fluorine and works both by direct attack and by decomposing into hydroxyl radicals. It deodorises, disinfects and decolourises without leaving a residue, but applied alone to leachate its utilisation efficiency is poor, its oxidising capacity is limited and the cost per point of degradation is high. That is why catalytic ozonation, with a solid catalyst in the bed, is the version usually specified. Liu Weihua et al. compared catalytic and plain ozonation on wastewater rich in refractory organics and found TOC and COD removal clearly better in the catalytic case.

Electrochemical oxidation runs at ambient temperature and pressure and needs no added reagent, so it avoids secondary pollution altogether. Lei et al. used it as advanced treatment after biology and removed 98.5% of COD, 99.9% of ammonia nitrogen and 99.9% of BOD. The mechanism splits between cathode reduction, which takes out most heavy metals, and anode oxidation, which mineralises organics. Plasma oxidation, supercritical water oxidation and photochemical routes are also in service. All of them are energy-hungry and none is fully mature, so the sensible engineering position is to pair them with cheaper stages, specify low-energy variants, or recover energy and material from the leachate and the landfill itself to drive the plant.

Read the whole picture and the sequence becomes obvious: biology for bulk carbon and nitrogen, MBR where solids must be held back hard, UASB where strength is high, and one oxidation stage at the tail for the fraction nothing else touches. The mistake is expecting any single box to do two jobs.