Biological oxidation of saline ammonia to nitrate, known as nitrification, takes place in two steps (Richard, 1978): initially conversion to nitrite by Nitrosomonas bacteria, followed by oxidation to nitrate by Nitrobacter bacteria. Both steps require oxygen. Carbon dioxide is the carbon source; 1 mg/l ammonia (as N) consumes about 7.2 mg/l alkalinity (as CaCO3). When treating some soft waters, therefore, alkalinity may have to be added. The process also requires about 0.3 mg/l phosphates (as PO4) (Lutle, 2013) to allow nitrifying bacteria to develop. The seeding period for this is about 1–2 months. The optimum pH range for the reaction is between 7.2 and 8.2. In practice, nitrification is almost 100% complete (Lytle, 2007). The optimum temperature for bacterial growth is in the range 25–30°C. The water temperature should be greater than 10°C; there is no biological activity below 4°C. The process requires oxygen at a rate of about 4.57 mg per mg of ammonia (as N) (Richard, 1978). At high ammonia concentrations, simple saturation of the water with oxygen by aeration may therefore prove inadequate and oxygen must be continually added.
In biological removal of ammonia in drinking water treatment, nitrification is usually carried out in rapid gravity filters. A maximum ammonia concentration of 1.5 mg/l as N can be removed in conventional rapid gravity filters, depending on the temperature and the dissolved oxygen concentration of the influent water. Filtration rates could be in the range 5–10 m3/h.m2 and, to remove 1 mg/l ammonia (as N), the necessary EBCT is about 20, 10 and 5 minutes at 5, 10 and 30°C, respectively. After nitrification, the water could be devoid of oxygen depending on the ammonia concentration and may need aeration. For higher ammonia concentrations BAFs have been used. These are similar in principal to the trickling and/or aerated filters used in sewage treatment where there is a continuous flow of air through the media of the filter bed. The ammonia loading of a BAF filter is typically 0.25–0.6 kg of ammonia (as N)/day.m3 (of media), depending on the type and effective size (d10) of media.
In drinking water treatment the BAF consists of a bed of coarse media (d10 between 1.5 and 3.0 mm); filtration is either downflow counter-current or upflow co-current with air flow injected continuously into the bottom of the filter bed using either an independent pipe lateral system or special nozzles in a plenum floor design. The volumetric ratio of air:water is in the range 0.3–1.0 (Degremont, 2007). Upflow filters are generally 15–25% more efficient than downflow filters; to remove 1 mg/l of ammonia as NH4+ (0.78 mg/l as N) at pH 7.2 and water temperature of 10°C, the EBCT required for upflow and downflow filters using d10 2 mm ‘Bioloite™’ (an expanded clay medium) is about 3 and 4 minutes, respectively. Pilot plant work has demonstrated that, based on first-order reaction kinetics, the reaction rate constant for upflow filter was 60% higher than that for the downflow filter (Heard, 2002). The depth of the filter medium would be a function of EBCT and filtration rate and is dependent on the raw water ammonia concentration: a water containing 2.5 mg/l ammonia (as N) would require a depth of about 2.5 m for an upflow filter compared to 3 m for a downflow filter. Upflow BAFs have a surface loading rate of 10–12 m3/h.m2 and a coarse medium (d10 1.5–2.0 mm) while downflow BAFs operate at surface loading rates of 8–10 m3/h.m2 (depending on the suspended solids loading) and use a coarser media. Such surface loading rates are feasible with expanded mineral filter media mostly of proprietary makes. For example, Filtralite of size 2.5–5.0 mm (d10 2.7 mm) removes about 90% of raw water ammonia using an EBCT of 12 minutes. Filters using naturally occurring media such as pozzolana or carbon (2–5 mm or even larger to give high specific surface per unit volume) operate at about 5 m3/h.m2 with EBCT of about 20–30 minutes (Lacamp, 1990).
BAFs are washed by concurrent application of air and water at about 16 and 4 mm/s, respectively followed by a high rate rinse at 12.5 mm/s. The minimum process air rate is about 0.8 mm/s. Washwater should be free of chlorine. When treating surface water BAFs are best used after the clarification stage and downflow filters are generally preferred. Upflow filters are generally used for groundwaters particularly those containing high ammonia concentrations that require high air:water flow rates and low turbidity. BAFs are best followed by conventional rapid gravity sand or anthracite–sand filtration in order to produce a water free of suspended matter. The biological process is adversely affected by chlorine, hydrogen sulphide, heavy metals and precipitates from iron and manganese oxidation and other suspended solids in the water. When ammonia is present together with iron and manganese, the order of biological removal is iron, ammonia followed by manganese (Mouchet, 1992). Iron is best removed separately by chemical oxidation. If the ammonia concentration is high manganese is not removed in the same filter unless adequate EBCT is provided. BAF beds also remove organic carbon effectively. Manganese dioxide coated sand filters have been successfully used to oxidize low concentrations of ammonia biologically and manganese by catalytic oxidation (Janda, 1994).
The biological reaction principle can also be applied to sedimentation tanks of the sludge blanket type. The sludge acts as the medium for the growth of nitrification bacteria. Oxygen for the nitrification reaction is limited to that which can be contained in the feed water and the ammonia removal is limited to about 0.5 mg/l as N.