Engineering Boundaries of Partial Nitritation and Anammox: Influent Thresholds, Loading Rates and Start-up Control Points

1. Current Status and Standard Boundaries (Read First)

At the time of writing, China has no formally published national or industry standard for the anammox wastewater treatment process. Two categories of public document can be cited, and their status must be stated honestly in design documents:

1. The All-China Environment Federation group standard Technical specification for anammox treatment of high ammonia wastewater, draft for comment, together with its explanatory memorandum. The memorandum states plainly that anammox technology is highly complex and unstable in operation, that no publicly available technical specification had previously been identified, and that a complete specification was urgently needed to guide practice.
2. The national guiding technical document Technical specification for anammox treatment of high strength ammonia nitrogen wastewater. According to the registration on the national public service platform for standards information, it is administered by the national technical committee for water reuse under the Standardization Administration, with drafting organisations including municipal drainage groups, universities and environmental enterprises. The drafting group was formed and held its kick-off meeting in October 2025, carried out surveys and case collection between October and December 2025, and planned to open the draft for public comment in February 2026.

All parameters below should therefore be read as design reference values from a group standard draft for comment, not as mandatory provisions. Engineering application must still be confirmed by a nitrogen removal rate pre-test.

2. Definitions and Process Configurations

Chapter 3 of the draft provides four definitions on which the parameter system rests:

• Clause 3.1, high ammonia wastewater: wastewater with an ammonia nitrogen concentration above 100 mg/L.
• Clause 3.2, one-stage partial nitritation anammox: partial nitritation and anammox integrated in a single reactor, with dissolved oxygen and other conditions controlled so that ammonia oxidising bacteria and anammox bacteria act synergistically to remove ammonium nitrogen and total nitrogen.
• Clause 3.3, two-stage partial nitritation anammox: the two reactions placed in separate reactors, partial nitritation in the first and anammox in the second.
• Clause 3.6 defines the control indicator delta nitrate-N over delta ammonium-N, the ratio of the change in nitrate nitrogen to the change in ammonium nitrogen across the anammox tank. Departure of this ratio from its theoretical value indicates excessive proliferation of nitrite oxidising bacteria.

Clauses 4.4 and 4.5 set out how to choose between the two configurations. The one-stage route preferably uses integrated fixed-film activated sludge, granular sludge, a biofilter or a moving bed biofilm reactor, and suits landfill leachate, sludge digester liquor, food waste fermentation liquor and semiconductor wastewater, with reasonably wide applicability. The two-stage route suits high ammonia wastewater containing inhibitors of anammox bacteria, but the partial nitritation stage readily accumulates nitrite and generates free nitrous acid inhibition, and the anammox stage imposes a fixed ratio of ammonium to nitrite in its feed, so the engineering design is more complex than for the one-stage route.

3. Influent Quality Thresholds

Table 1 of clause 4.2 gives the key influent indices, which form the first screen at scheme stage:

• One-stage configuration: ammonium nitrogen not less than 100 mg/L; COD to TKN ratio not greater than 3; temperature 26 to 38 degrees Celsius; pH 6.7 to 8.5; suspended solids not more than 500 mg/L; salinity not more than 30000 mg/L; alkalinity not less than four times the ammonium nitrogen concentration.
• Two-stage, partial nitritation: ammonium nitrogen not less than 100 mg/L; pH 6.7 to 8.5; suspended solids not more than 1000 mg/L; alkalinity not less than four times the ammonium nitrogen concentration.
• Two-stage, anammox: COD to TKN not greater than 2; temperature 26 to 38 degrees Celsius; pH 6.7 to 8.5; suspended solids not more than 500 mg/L.

Clause 4.3 requires pre-treatment where Table 1 is not met, with the wastewater entering the anammox system only after compliance, and a nitrogen removal rate pre-test where necessary. Clause 3.4 defines that pre-test as treating the target high ammonia wastewater with partial nitritation and anammox sludge before design, to measure the ammonia and total nitrogen removal rates and other key design parameters. This matters especially for industrial projects, where inhibitory substances differ greatly between sectors and transferring experience from one industry to another carries high risk.

4. Pre-treatment Design Requirements

Chapter 5.1 gives parameters that can be applied directly:

• Clause 5.1.1, flow and quality equalisation: an equalisation tank should be provided upstream of the biological process, with functions for flow, quality, temperature, acid and alkali equalisation and emergency storage, and a retention time of not less than 24 hours; the outflow should be uniform and continuous; anti-settling facilities should be provided along with an overflow and an emptying facility.
• Clause 5.1.2, suspended solids: where the influent exceeds the Table 1 limit, high-rate clarification such as lamella or side-flow or horizontal tube settlers, or flotation, should be used; effluent suspended solids from coagulation, sedimentation or flotation should be 100 to 500 mg/L; polyaluminium chloride or polymeric ferric sulphate are the preferred coagulants with polyacrylamide as flocculant aid; settlers should have automatic flushing with a cycle of 24 to 48 hours, scheduled at low flow periods.
• Clause 5.1.3, organic carbon: where COD to TKN exceeds 3 the organic matter should be pre-treated; where COD to TKN exceeds 3 and COD is below 3000 mg/L an activated sludge pre-treatment is preferred; where COD to TKN exceeds 3 and COD is 3000 mg/L or more anaerobic digestion pre-treatment is preferred.
• Clause 5.1.4, alkalinity and pH: where the influent pH is below 6.7 or the alkalinity is below four times the ammonium nitrogen concentration, alkalinity must be supplemented, preferably with sodium bicarbonate, sodium carbonate or sodium hydroxide.
• Clause 5.1.5: the ambient temperature for the anammox reaction should be controlled between 26 and 38 degrees Celsius.

5. Design Parameter Values

5.1 One-stage configuration

Clause 5.2.2 sets the design conditions for the anammox tank:

• The substrates are nitrite nitrogen and ammonium nitrogen; the influent ammonium nitrogen should not fall below 100 mg/L and the nitrite nitrogen in the tank should not exceed 100 mg/L.
• The influent COD to TKN ratio must not exceed 3; where it does, biodegradable COD must be removed in pre-treatment before entry.
• Influent suspended solids must not exceed 500 mg/L; tank pH should be 6.7 to 8.5; water temperature should be 26 to 38 degrees Celsius with heating or cooling where this is not met; influent salinity should be below 30000 mg/L.
• Dissolved oxygen: 0.1 to 1.2 mg/L for integrated fixed-film activated sludge, rising to 0.8 to 2.0 mg/L for granular sludge and biofilm processes, determined from the nitrite concentration and the activity of the anammox bacteria.
• The total nitrogen removal loading rate should be designed at 0.2 to 1.0 kg total nitrogen per cubic metre per day.

5.2 Two-stage, partial nitritation tank

Clause 5.3.2: influent suspended solids should be kept below 1000 mg/L; temperature should be 26 to 38 degrees Celsius with heating, insulation or cooling as required; influent pH should be 6.7 to 8.5 with an alkali dosing system where the alkalinity to ammonia ratio falls below 4; dissolved oxygen should be 0.2 to 1.0 mg/L; influent salinity should be below 30000 mg/L; where suspended or fixed carriers are installed the carrier filling ratio should be 20 to 50 percent; the ammonia sludge loading should be 0.1 to 0.5 kg nitrogen per kg volatile suspended solids per day and the ammonia volumetric loading 0.2 to 2.0 kg nitrogen per cubic metre per day.

5.3 Two-stage, anammox tank

Clause 5.3.3: influent suspended solids not more than 500 mg/L; influent COD to TKN preferably below 3; water temperature 26 to 38 degrees Celsius with heating or cooling where this is not met; pH controlled at 6.7 to 8.5 with acid or alkali dosing where necessary; the biomass is present mainly as granular sludge and biofilm, with a carrier filling ratio of 20 to 50 percent where carriers are used; the total nitrogen removal loading rate should be designed at 0.5 to 5.0 kg nitrogen per cubic metre per day.

The comparison is instructive: the upper bound for the two-stage anammox tank, 5.0, is far higher than the one-stage figure of 1.0, because decoupling the two reactions lets each run at its optimum. The price is the need to hold the nitrite to ammonium ratio of the partial nitritation effluent within tight limits and a higher risk of free nitrous acid inhibition. Selection should not be based on the loading ceiling alone.

6. Inoculation and Start-up

6.1 One-stage configuration

Clause 7.2 gives a set of window values that can be written directly into a commissioning plan:

• Clause 7.2.1: the tank should be seeded with conventional activated sludge together with anammox sludge, the latter as biofilm, granular sludge or flocculent sludge.
• Clause 7.2.2: after seeding with conventional activated sludge, nitrite oxidising bacteria should be progressively washed out to establish ammonia oxidising bacteria as the dominant nitrifying population, with the nitrite accumulation ratio in the effluent reaching 10 to 50 percent.
• Clause 7.2.3: the anammox bacteria should account for 10 to 20 percent of the seeded anammox sludge; where granular or flocculent sludge is seeded the mixed liquor suspended solids should exceed 1000 mg/L; where biofilm is seeded, the seeded carriers should exceed 10 percent of the total carrier inventory.
• Clause 7.2.4: during commissioning the dissolved oxygen should be 0.1 to 1.0 mg/L for integrated fixed-film activated sludge and may be raised to 0.5 to 2.0 mg/L for granular sludge and biofilm processes.
• Clause 7.2.5: effluent ammonium nitrogen should be 30 to 80 mg/L, effluent nitrite nitrogen should not exceed 50 mg/L, and the nitrate to ammonium delta ratio should not exceed 15 percent.
• Clause 7.2.6: effluent pH should not be below 6.7 and effluent alkalinity should not be below 200 mg/L.
• Clause 7.2.7: the anammox system generally does not waste sludge deliberately; where it is necessary, the sludge retention time should be 50 to 100 days.

The explanatory memorandum adds that partial nitritation must be started first and the anammox components, granular sludge or suspended carriers seeded only once the nitrite accumulation ratio reaches 50 percent, with the nitrite concentration in the tank at seeding not exceeding 50 mg/L.

6.2 Two-stage configuration

• Clause 7.3.1: the partial nitritation tank should be seeded with conventional activated sludge to 4000 to 6000 mg/L mixed liquor suspended solids; the anammox tank should be seeded with biofilm carriers at a filling ratio above 10 percent or with granular sludge to at least 1000 mg/L.
• Clause 7.3.2: commissioning should start with the partial nitritation tank, moving on to the anammox tank only when its feed requirements are met; the nitrite to ammonium ratio of the partial nitritation effluent should be controlled at about 1.32 to 1, achievable through hydraulic retention time, aeration rate or feed distribution.
• Clause 7.3.3: the partial nitritation tank should hold dissolved oxygen at 0.2 to 1.0 mg/L, free ammonia at 5 to 35 mg/L and free nitrous acid at 0.1 to 1.0 mg/L, with effluent alkalinity not below 200 mg/L.
• Clause 7.3.4: mixed liquor volatile suspended solids in the partial nitritation tank should be maintained at not less than 1000 mg/L, with activated sludge added promptly where it falls below.
• Clause 7.3.5: where the partial nitritation tank runs as suspended or granular sludge, the sludge retention time should be held at not less than 10 days.
• Clause 7.3.6: the anammox tank should be started by raising the nitrogen loading progressively until the design total nitrogen removal rate is reached.
• Clause 7.3.7: nitrite in the anammox tank should be kept below 100 mg/L, free ammonia not above 35 mg/L and temperature at 26 to 38 degrees Celsius.
• Clause 7.3.8: anammox bacteria are anaerobes, so dissolved oxygen in the anammox tank must be held below 0.5 mg/L.
• Clause 7.3.9: granular sludge and biofilm anammox processes generally do not waste sludge and the sludge retention time is not controlled; where wasting is necessary, biomass retention equipment must be installed to prevent loss of the culture.

7. Monitoring and Ancillary Works

Clauses 7.2.8 and 7.3.10 give sampling points and monitoring items in Tables 2 and 3. The overall inlet and outlet are monitored for COD, BOD5, suspended solids, total Kjeldahl nitrogen, ammonium, nitrite, nitrate, total phosphorus, orthophosphate, pH, alkalinity, temperature and salinity; the two-stage scheme adds temperature, pH, dissolved oxygen, total Kjeldahl nitrogen, ammonium, nitrite and nitrate at the partial nitritation tank; and the reactor sludge is monitored for mixed liquor suspended solids, mixed liquor volatile suspended solids, settled volume, sludge volume index and micro-fauna. Clause 7.1.5 requires monitoring practice and analytical methods to comply with HJ/T 92.

Chapter 8 covers ancillary works, of which the blower house is the most directly relevant: clause 8.2.1 requires noise control; clause 8.2.2 recommends an independent foundation for each blower; clause 8.2.3 requires duty and standby blowers; and clause 8.2.4 requires a return air main with valves and a non-return valve to prevent backflow, with the blower house designed to GB 50014. Chapter 9 requires collected odorous gas to meet GB 14554 after treatment and site noise limits to meet GB/T 50087.

8. Benefit Claims and the Real Engineering Boundaries

Chapter 7 of the explanatory memorandum states the technical advantage as follows: compared with conventional biological nitrogen removal, nitrogen removal efficiency is raised by a factor of 2 to 4, aeration energy consumption is cut by 60 percent, nitrogen removal chemicals are eliminated entirely, footprint is reduced by more than 30 percent, sludge production falls by more than 90 percent and greenhouse gas emissions drop by more than 90 percent. These are sector-level assessments in a standard drafting document, a summary by the drafting group of the technology's overall advantage; they are not guaranteed values for a specific project, and project approval should rely on the project's own nitrogen removal rate pre-test and on measured data from comparable plants.

The practical boundaries can be summarised in four points:

1. The target is high ammonia wastewater, meaning ammonia nitrogen above 100 mg/L, including municipal landfill leachate, food waste fermentation liquor, sludge digester liquor and faecal wastewater, agricultural livestock wastewater and its fermentation liquor, and industrial food, pharmaceutical, coking, chemical and semiconductor wastewater. Direct application to low-temperature, low-ammonia mainstream municipal wastewater still lacks standard support.
2. Temperature is a hard constraint. Both the design document and the operating procedures treat 26 to 38 degrees Celsius as the reaction temperature band, with heating or cooling mandatory outside it, so the energy account in northern regions must be calculated separately.
3. The carbon to nitrogen ratio is a hard constraint. Where COD to TKN exceeds 3 for the one-stage route or 2 for the two-stage anammox tank, biodegradable COD must first be removed, otherwise heterotrophs will outcompete anammox bacteria and occupy the biofilm.
4. Inhibitors are the main uncertainty. Salinity is capped at 30000 mg/L and the partial nitritation stage avoids inhibition within a free ammonia window of 5 to 35 mg/L and a free nitrous acid window of 0.1 to 1.0 mg/L, but characteristic inhibitors such as antibiotics, phenols and heavy metals have no uniform threshold and can only be established by a nitrogen removal rate pre-test.