Treatment of High Ammonium–Nitrogen Wastewater from Composting Facilities by Air Stripping and Catalytic Oxidation

Abstract

Composting municipal wastewater sludge may generate composting wastewater (acid washer water and tunnel wastewater) with high ammonium–nitrogen (NH₄–N) concentration; this kind of wastewater is usually generated in a rather small daily amount. A procedure of air stripping with catalytic oxidation was developed and tested with pilot-scale and full-scale units for synthetic disposal of the high NH₄–N wastewaters from composting facilities. In air stripping, around 90% NH₄–N removal efficiency was reliably achieved with a maximum of 98%. A model to describe the stripping process efficiency was constructed, which can be used for process optimization. After catalytic oxidation, the concentrations in the outlet gas were acceptable for NH₃, NO_X, NO₂, and N₂O, but the NH₃ and N₂O concentrations limited the feasible loading range. The treatment costs were estimated in detail. The results indicate that air stripping with the catalytic oxidation process can be applied for wastewater treatment in composting facilities.

Appendix

1.1 Cost estimation

The cost estimation is based on a credible assumption as the air-stripping system is running consistently at the same level of overall efficiency. The utilities of electricity, heating oil, and biogas as energy sources are compared from the point of economy. Biogas sources are distinguished as commercial purchase and cost-free supply from the close landfills or anaerobic digestion facilities. The latter is thought of as free in the cost estimation. To avoid complication and interference with individual cases, the air-stripping process is pre-digested, as illustrated in Fig. 10, and operational costs are defined mainly as chemicals cost and energy cost. It is assumed that 50% thinner and 50% heat are recycled between the sequential two batches. Two parallel stripping reactors are used.

Fig. 10

Simplified diagram of air-stripping process

Based on titration tests, the relationship of pH and chemical cost is defined as

$$ {C_{\text{CN}}} = 2\tau \times \left( {{\text{1,418 p}}{{\text{H}}^2} - {\text{21,646 pH}} + 82,561} \right) \times \frac{\text{UP}}{N_i}, $$

(2)

where

C _CN :

cost of alkali addition with NH₄–N loading (EUR/kg NH₄–N loading)

UP:

unit price of alkali (EUR/L)

pH:

pH value expected

τ :

process coefficients, usually 1.05

N _i :

the initial NH₄–N concentration in composting wastewaters (mg/L)

Equation 2 is only applicable for the expected pH range of 9.3–9.7 and initial NH₄–N concentration range of 2,500–10,000 mg/L. If the required pH were above this range, the cost would be almost double.

Based on Fig. 1, the energy cost is summarized as the energy needed for heating of acid washer water/stripping air and for aeration/alkali-feed pumps. Other daily energy consumptions such as lighting are not included in this estimation. Heater energy-use efficiency is assumed as 78%. Electricity, oil, and biogas are potential energy sources to estimate. Energy cost is expressed as

$$ {C_{\text{EN}}} = \left[ {\frac{{2,8 \times {{10}^{ - 4}}D \times {c_{\text{p}}}\left( {{T_{\text{e}}} - {T_{\text{i}}}} \right)}}{{{N_{\text{I}}}}} + \frac{{2,8 \times {{10}^{ - 4}}{D_{\text{a}}} \times {c_{\text{pa}}} \times {\text{DI}} \times R\left( {{T_{\text{ea}}} - {T_{\text{ia}}}} \right)}}{{{N_{\text{I}}}}} + \frac{{{{10}^3}\left( {{E_{\text{A}}} + {E_{\text{P}}}} \right)}}{{Q \times {N_{\text{I}}}}}} \right]*{\text{UP,}} $$

(3)

where

C _EN :

cost of energy with NH₄–N loading (EUR/kg NH₄–N loading)

D :

density of composting wastewaters (kg/m³)

c _p :

specific heat capacity of composting wastewaters (J/kg°C)

Q :

volume of air stripper batch (m³)

T _e :

temperature expected for composting wastewaters (°C)

T _i :

temperature initial for composting wastewaters (°C)

D _a :

density of air (kg/m³)

c _pa :

specific heat capacity of air (J/kg°C)

DI:

dilution factor

R :

ratio air to liquid volume, assumed as 2,400

T _ea :

temperature expected for air (°C)

T _ia :

temperature initial for air (°C)

E _A :

energy for aeration (kWh)

E _p :

energy for alkali feed pump (kWh)

UP:

unit price of energy (EUR/kWh)

In these studied cases, composting wastewater (acid washer water) is usually generated only in small daily amounts. The requirement of capacity of the stripping system is not an acute factor. It is suggested that it is better to design the volume of air stripper batch (Q) in the range of 2 to 10 m³/day to achieve the optimal performance–price ratio.

The operational cost including chemicals cost and energy cost is calculated with Eq. 4.

$$ {C_{\text{N}}} = \left( {{C_{\text{CN}}} + {C_{\text{EN}}}} \right) \times f, $$

(4)

where

C _N :

operational cost with NH₄–N loading (EUR/kg NH₄–N loading)

F :

maintenance and loss coefficients, usually 1.05–1.1

Assumption: in a typical pilot-scale air-stripping process, a 9-kW air blower is used for aeration. Two 0.25-kW alkali-feed pumps are used for alkali feeding. The specification and cost estimation of the process are summarized in Table 6.

Table 6 Specification and cost estimation for operation and capital of an air-stripping process for acid washer water

The capital and labor costs are estimated as well, based on EU Member States' conditions. The assumptions are that two 17.5-m³ plastic reactors, one catalytic VOC incinerator with a capacity of 1,000 m³/h, one air blower with a capacity of 1,000 m³/h, one heater and heat exchanger, and two alkali-feed pumps are the essential equipment in this system. Costs of the necessary monitoring systems including pH, temperature, and airflow probes and the accessories including valves and pipes are estimated too.

For a manual monitoring system, portable gas meters and weekly manual work of 8 h are used in the estimation. The estimated capital cost is EUR 120,000 and the labor cost is EUR 6,000/year. If an automatic monitoring system is used and online gas analyzers and probes are combined with the computer control system, the daily manual work will decrease considerably (to half an hour). The estimated capital cost is EUR 125,000, and the labor cost is EUR 2,000/year.

As a whole, in stripping, part of the capital costs were estimated as EUR 0.4/kg N and operation cost EUR 1.6–3/kg N removed, whereas in catalytic oxidation part, the capital costs were estimated as EUR 1/kg N and operation cost EUR 0.2/kg N removed.