### 1. Needs for Transdisciplinary Approach for Environmental Sustainability

### 2. Natural Systems and Pollution Control

### 2.1. Waste Stabilisation Ponds

_{2}) gas, according to Eq. (1).

##### (1)

$${\text{NH}}_{3}+7.62{\text{CO}}_{2}+2.53{\text{H}}_{2}\text{O}\underset{\text{ALGAE}}{\overset{\text{Sunlight}}{\to}}\underset{(\text{New\hspace{0.17em}algal\hspace{0.17em}cells})}{{\text{C}}_{7.62}{\text{H}}_{8.06}{\text{O}}_{2.53}\text{N}}+7.62{\text{O}}_{2}$$_{2}gas produced in Eq. (1) is then used by suspended bacteria to oxidise the incoming organic waste, as shown in Eq. (2).

##### (2)

$$\underset{(\text{Organic\hspace{0.17em}Waste})}{{\text{C}}_{5}{\text{H}}_{7}{\text{O}}_{2}\text{N}+5{\text{O}}_{2}}\stackrel{\text{bacteria}}{\to}5{\text{CO}}_{2}+{\text{NH}}_{3}+2{\text{H}}_{2}\text{O}+\text{energy}$$_{2}above that required by the bacteria, an aerobic environment is maintained. Thus it can be seen that there is interdependence between algae and bacteria, with the algae supplying oxygen required by the bacteria, and the bacteria making available the CO

_{2}and inorganic nutrients required by the algae. Effluent of facultative ponds normally contains low dissolved organic matter, but high content of algal and bacterial cells which can be removed further in maturation or polishing ponds connected in series to the facultative ponds.

### 2.2. Aquatic Weeds

*Eichhomia crassipesy,*duckweeds (

*Woifia arrhiga*) and water lettuce (

*Pistia stratiotes*). Waste stabilisation in an aquatic weed pond is accomplished through the reactions listed in Eqs. (1) and (2), except O

_{2}gas in Eq. (1) is produced by the photosynthetic activity of the aquatic weeds and the produced O

_{2}is transferred to the roots and stems for the biofilm bacteria to biodegrade the organic matter (Eq. (2)). Reported values of O

_{2}release rates from the roots of emergent plants are 5–45 g O

_{2}/m

^{2}/day, with a mean value of 20 g O

_{2}/m

^{2}/day. Because there will be less algae growing in aquatic weed ponds, the aquatic weed pond effluent should contain less suspended solids and low organic matter content, suitable for discharging into water bodies or further reuse.

### 2.3. Constructed Wetlands

*Typha*), bulrushes (

*Scirpusy*) and reeds (

*Phragmites*) are the major and typical component of the wetland systems. Wetland is a natural system where complex physical, chemical and biological reactions essential for waste-water treatment, exist. Constructed wetlands can range from creation of a marshland to intensive construction involving earth moving, grading, impermeable barriers or erection of tanks or trenches [7].

### 3. Natural Systems and Waste Recycling

### 3.1. Algal Protein Production

*μ*m) which causes difficulty in the harvesting of algal cells from the HRAP effluent and concentrating them in, to paste or pellet forms suitable for use as animal or human feeds. The available harvesting technologies, such as micro-straining, belt filtration and centrifugation are still not cost-effective and more research in this area is needed. Most algal species, except

*Spirulina*, have rigid cell walls not easily digestible by non-ruminants and the high nucleic acid content may cause some health effects if ingested in large quantity.

### 3.2. Fish Production

### 3.3. Biomass Production

^{2}(wet weight). For a pond with a surface area of 1 ha, if 50% of the water hyacinth plants are harvested once in 2 weeks, the amount of water hyacinth biomass available is 200 tons/mon or the biomass productivity of water hyacinth is 200 tons/ha/mon. Because aquatic weeds contain about 95% of water content, the dry biomass productivity of water hyacinth is approximately 120 tons/ha/yr (or annual protein production of 20 tons/ha/yr).

### 4. An Integrated Kinetic Model for the Design of Natural Systems

### 4.1. The Need for an Integrated Kinetic Model

### 4.2. Current Design Models

_{e}, and C

_{0}, are the effluent and influent substrate concentration, and t is the hydraulic retention time.

##### (6)

$$\frac{{\text{C}}_{\text{e}}}{{\text{C}}_{0}}=\frac{4{\text{a}}_{1}{\text{e}}^{\frac{1}{2\text{d}}}}{{(1+{\text{a}}_{1})}^{2}{\text{e}}^{\frac{{\text{a}}_{1}}{2\text{d}}}-{(1-{\text{a}}_{1})}^{2}{\text{e}}^{\frac{{\text{a}}_{1}}{2\text{d}}}}$$### 4.3. Concept of the Integrated Kinetic Model

_{s}is liquid sublayer thickness, L

_{f}is thickness of biofilm, C

_{w}is substrate concentration in bulk water, C

_{s}is substrate concentration at the liquid sublayer and biofilm interface, and C

_{f}is the substrate concentration in biofilm.

##### (8)

$$\text{d}\frac{{\text{d}}^{2}{\text{C}}_{\text{w}}}{{\text{dz}}^{2}}=\frac{{\text{dC}}_{\text{w}}}{\text{dz}}+\text{tr}+{\text{ta}}_{\text{s}}\text{J}$$^{2}/day), r is substrate utilisation rate by suspended microbes (g/m

^{3}/day), as is specific surface area of the biofilm per unit volume of the treatment unit (m

^{2}/m

^{3}), and other terms are defined previously.

##### (9)

$$\text{d}\frac{{\text{d}}^{2}{\text{C}}_{\text{w}}}{{\text{dz}}^{2}}=\frac{{\text{dC}}_{\text{w}}}{\text{d}}+\text{t}\left({\text{k}}_{\text{fs}}+{\text{a}}_{\text{s}}\frac{\alpha \beta}{\alpha +\beta}\right){\text{C}}_{\text{w}}$$*φ*is characteristic biofilm parameter, D

_{w}is diffusion coefficient in liquid sublayer (m

^{2}/day), D

_{f}is diffusion coefficient in biofilm (m

^{2}/day), k

_{fs}is first-order rate constant of suspended bacteria (1/day), and k

_{fa}is first-order rate constant of biofilm (1/day).

_{w}=C

_{0}at z=0 and dC

_{w}/dz=0 at z=1.

##### (10)

$$\frac{{\text{c}}_{\text{e}}}{{\text{C}}_{0}}=\frac{2{\text{a}}_{1}{\text{e}}^{\frac{1}{2\text{d}}}}{(1+{\text{a}}_{1}){\text{e}}^{\frac{{\text{a}}_{1}}{2\text{d}}}-(1-{\text{a}}_{1}){\text{e}}^{-\frac{{\text{a}}_{1}}{2\text{d}}}}$$_{e}and C

_{0}are the effluent and influent substrate concentrations respectively, and

##### (14)

$$\text{p}=\frac{{\text{a}}_{\text{s}}{\text{k}}_{\text{fb}}{\text{C}}_{\text{w}}*100}{{\text{k}}_{\text{fs}}{\text{C}}_{\text{w}}+{\text{a}}_{\text{s}}{\text{k}}_{\text{fb}}{\text{C}}_{\text{w}}}$$##### (15)

$$\text{p}=\frac{100}{1+\frac{{\text{k}}_{\text{fs}}}{{\text{k}}_{\text{fb}}{\text{a}}_{\text{s}}}}$$### 4.4. Application to WSP and Attached-growth WSP

_{5}concentrations with the observed BOD

_{5}concentrations. The model was able to predict the effluent BOD

_{5}concentrations of these two ponds reasonably well, indicating the significance of these biofilm bacteria in organic matter degradation in facultative ponds (Fig. 6).

### 4.5. Application to Water Hyacinth Ponds

_{5}values were found in close agreement with the observed effluent values (Fig. 8). Based on the model, Polprasert and Khatiwada [34] found the contribution of the biofilm bacteria attached to the roots of the water hyacinth plants in the WHPs directly related to the improvement on the effluent quality when compared to a similarly operated WSP as is demonstrated in Fig. 9. A design chart in Fig. 7 based on Eq. (10) can be used for WHPs, whereas other design considerations are summarized in Table 8 [4].

### 4.6. Application to Duckweed Ponds

_{5}, NH

_{3}-N, and TN can be applied in the duckweed ponds, while hydraulic retention time, organic loading rate, and stocking density are varied during operation period. The observed values from pilot-scale experiment at the Wageningen University, Netherland and the Birzeit University, Palestine and the integrated kinetic model finding developed from experiment at the Asian Institute of Technology, Bangkok illustrate high correlation [35].

### 4.7. Application to Constructed Wetlands

^{2}, 10 cm water depth and 60 cm media depth data and with the aid of the integrated kinetic model, the effective specific surface area, as, was found to be approximately 4.40 m

^{2}/m

^{3}(Fig. 10). Fig. 11 shows that the integrated kinetic model could predict organic matter (COD) removal in a FWS constructed wetland satisfactorily. A design chart in Fig. 7 based on Eq. (10) can be used for constructed wetlands, whereas other design considerations are summarized in Table 9 [36].