Based on the cost function of constructing centralized and decentralized wastewater reclamation and reuse projects,a critical distance model was developed.

In this paper, the theorietical critical distance of free space model is discussed, the critical distance formula is deducted, and the physical meaning and applications of the critical distance are analysed.

According to cost function,the author provides that in the urban planning area,taking critical distance as economical criterion of municipal wastewater\'s centralized or decentralized treatment can define a reasonable municipal waster treatment plant layout and numbers needed more easily from the point of cost,so as to improve the rationality and economy of municipal wastewater planning.

The author expatiates on electrical wire stress and sag and their derivation in theory,describes the way of distinguishing critical span and determining weather control conditions,and deduces formulae of stress,sag and critical span of seriate span and isolated span used in the design.

The paper has analyzed the variation regulations of the critical spans when the different values are taken as the safety coefficient N for the "experience method" and discovered a new way to determine the maximum working stress σ M of conductor with the "calculation method".

The final output signal-to-noise ratio of the electronic intelligence receiver is obtained at the critical range,and compared with the signal-to-noise ratio required for the energy detection.