Constructed Wetland Design: Parameter Boundaries and Clogging Control in HJ 2005-2010

1. Scope first, parameters second

HJ 2005-2010, Technical specification of constructed wetlands for wastewater treatment engineering, was approved by the former Ministry of Environmental Protection on 2010-12-17 and became effective on 2011-03-01 as a first edition. Clause 1 sets a narrow scope: it applies to municipal domestic sewage, effluent from municipal wastewater treatment plants, and other wastewater of similar character, and serves as the technical basis for design, construction, environmental protection acceptance and subsequent operation and maintenance. Industrial wastewater therefore cannot be designed by simply copying this specification.

Scale is classified in clause 5.1.2: below 3000 m3/d is small, 3000 to 10000 m3/d is medium, and 10000 m3/d and above is large; the text states that the lower bound is inclusive and the upper bound exclusive. For site selection, clause 5.3.4 prefers depressions or ponds with a natural slope of 0% to 3% and unused land, while clause 5.4.2 requires multi-unit systems to follow the natural slope and use gravity flow, with a single lift where pumping is unavoidable.

2. Influent limits: the three tiers of Table 1

Clause 4.2.3 requires the influent to meet Table 1, which is the first gate to check before design:

• Surface flow wetland: BOD5 not above 50 mg/L, CODCr not above 125 mg/L, SS not above 100 mg/L, ammonia nitrogen not above 10 mg/L, total phosphorus not above 3 mg/L;
• Horizontal subsurface flow wetland: BOD5 not above 80 mg/L, CODCr not above 200 mg/L, SS not above 60 mg/L, ammonia nitrogen not above 25 mg/L, total phosphorus not above 5 mg/L;
• Vertical subsurface flow wetland: BOD5 not above 80 mg/L, CODCr not above 200 mg/L, SS not above 80 mg/L, ammonia nitrogen not above 25 mg/L, total phosphorus not above 5 mg/L.

Only SS differs between the two subsurface configurations, 60 versus 80 mg/L. Subsurface flow is more sensitive to suspended solids, and clause 11.2.4 explains why: clogging is the dominant operational risk.

Clause 4.3 and Table 2 give removal efficiency ranges for use when measured data are absent: surface flow 40% to 70% for BOD5, 50% to 60% for CODCr, 50% to 60% for SS, 20% to 50% for ammonia nitrogen and 35% to 70% for total phosphorus; horizontal subsurface flow 45% to 85%, 55% to 75%, 50% to 80%, 40% to 70% and 70% to 80% respectively; vertical subsurface flow 50% to 90%, 60% to 80%, 50% to 80%, 50% to 75% and 60% to 80%. Nitrogen removal is clearly weakest in surface flow, while phosphorus removal is strongest in horizontal subsurface flow.

3. The main design parameter table and a numbering inconsistency

Clause 6.4.1.1 fixes the calculation order: the wetland area shall be determined by the BOD5 surface organic loading and shall simultaneously satisfy the hydraulic loading requirement. Clause 6.4.1.2 adds that the main design parameters should preferably be derived from test data, and that in the absence of such data empirical values or "Table 4" may be used - yet the table that follows is actually numbered Table 3. This is an editing inconsistency in the issued text, and citations should follow the table itself. Table 3 reads:

• Surface flow: BOD5 loading 15 to 50 kg/(ha.d), hydraulic loading below 0.1 m3/(m2.d), hydraulic retention time 4 to 8 d;
• Horizontal subsurface flow: BOD5 loading 80 to 120 kg/(ha.d), hydraulic loading below 0.5 m3/(m2.d), hydraulic retention time 1 to 3 d;
• Vertical subsurface flow: BOD5 loading 80 to 120 kg/(ha.d), hydraulic loading below 1.0 m3/(m2.d) with recommended values of 0.2 to 0.5 in northern China and 0.4 to 0.8 in southern China, hydraulic retention time 1 to 3 d.

The BOD5 loading unit is kg per hectare per day, so conversion to square metres requires division by 10000 - a frequent source of order-of-magnitude errors. The 2 to 8 fold difference in BOD5 loading and 5 to 10 fold difference in hydraulic loading between surface flow and subsurface flow explain why a surface flow system needs several times more land for the same flow.

4. Geometry: separate rules for subsurface and surface flow

Clause 6.4.2.1 for subsurface flow: horizontal subsurface flow cell area preferably below 800 m2 and vertical subsurface flow cell area below 1500 m2; length to width ratio preferably below 3:1; regular cell length preferably 20 to 50 m, with irregular cells requiring attention to uniform distribution and collection; water depth preferably 0.4 to 1.6 m; hydraulic slope preferably 0.5% to 1%.

Clause 6.4.2.2 for surface flow: length to width ratio preferably 3:1 to 5:1, and where the ratio exceeds 10:1 a dead zone curve must be calculated; water depth preferably 0.3 to 0.5 m; hydraulic slope preferably below 0.5%.

On distribution, clause 6.4.3.2 gives a quantifiable detail: perforated pipe orifice spacing should not exceed 10% of the cell width, orifice density should be uniform, and pipe length should be roughly equal to the cell width. Clause 6.4.3.3 requires the media around the perforated pipe to have a particle size larger than the orifice diameter.

5. Media, plants and liner

Clause 6.4.5.4: the initial porosity of the subsurface flow media layer should be controlled at 35% to 40%; clause 6.4.5.5 requires the media depth to exceed the deepest reach of plant roots. These two values are the starting point for porosity decay calculations, since clogging progressively shifts the actual retention time away from the design value.

Clause 6.4.6.6 gives planting density: 9 to 25 plants per m2 for emergent plants and 3 to 9 plants per m2 for both floating and submerged plants. Clause 6.4.6.1 warns that where the effluent discharges directly to a river or lake, invasive species such as water hyacinth must be chosen with caution. Clause 6.4.6.8 specifies planting soil as soft clay to loam, 20 to 40 cm thick, with a permeability of 0.025 to 0.35 cm/h.

Clause 6.4.7.1: the bottom and sides must be lined, and the liner permeability shall not exceed 1x10-8 m/s; clause 6.4.7.2 permits clay layers, polyethylene film and other waterproofing materials, referring to CJJ 17. Clause 10.2.6 adds that polyethylene film must be welded by qualified personnel with dedicated equipment and tested for permeability afterwards.

6. Four triggers for pretreatment

Section 6.3 turns the pretreatment decision into four quantitative tests:

• Clause 6.3.2: where BOD5/CODCr is below 0.3, hydrolysis acidification should be used;
• Clause 6.3.3: where SS exceeds 100 mg/L, a sedimentation tank should be provided;
• Clause 6.3.4: where oil content exceeds 50 mg/L, oil removal equipment should be provided;
• Clause 6.3.5: where DO is below 1.0 mg/L, aeration should be provided.

7. Two operational priorities: cold weather and clogging

Clause 11.2.3 gives three measures for low temperature operation: insulation to keep water temperature not below 4 degrees C; periodic measurement of frost penetration depth; and enhanced pretreatment to reduce the wetland load.

Clause 11.2.4 gives four anti-clogging measures for subsurface flow: control the suspended solids concentration entering the system; start desludging periodically, which corresponds to clause 6.4.4.1 requiring a desludging device at the bottom of subsurface flow cells; adopt intermittent operation where appropriate; and replace media locally. Clause 11.2.1 treats water level adjustment as a corrective tool: it may be used when short-circuiting occurs, but must not cause backing up at the inlet end or submergence at the outlet end.

Monitoring frequency is fixed in clause 7.2.1: rainfall and wetland water level once per day, plant density once per year; routine influent and effluent items include flow, water level, temperature, DO, pH, SS, BOD5, CODCr, ammonia nitrogen, nitrate and total phosphorus.

8. What the standard leaves to testing

Three gaps must be stated honestly. First, Table 3 is explicitly positioned as empirical values for use when test data are absent, because clause 6.4.1.2 gives priority to test data. Second, media gradation, the dosage of functional media and the resulting gain in phosphorus removal are not quantified; clause 6.4.5.3 merely recommends functional media where effluent nitrogen and phosphorus requirements are stringent. Third, desludging intervals and the proportion of media to be replaced are left at the words periodic and local, so the actual frequency must be derived from operating records.

The practical conclusion is to treat a constructed wetland as a low energy polishing unit rather than a universal secondary treatment process: enforce the Table 1 influent gate first, size the area with Table 3, and then write liner integrity, desludging and water level management into the operating manual. That sequence is what HJ 2005-2010 actually requires.