for example when i write:

partial model WasteWater.ASM1.Interfaces.ASM1base
  "Base class of WWTP modelling by ASM1"
  extends Interfaces.stoichiometry;
  package WWU = WasteWater.WasteWaterUnits;
 
  // parameters based on the original ASM1 publication based on 15 deg C   
  Real mu_h "Maximum heterotrophic growth rate f(T) [day^-1]";
  Real b_h "Heterotrophic decay rate f(T) [day^-1]";
  Real mu_a "Maximum autotrophic growth rate f(T) [day^-1]";
  //Real K_nh "Half-saturation (auto. growth) f(T) [g NH-N/m3]";
  Real b_a "Autotrophic decay rate f(T) [day^-1]";
  Real k_a "Ammonification rate f(T) [m3/(g COD day)]";
  Real k_h "Maximum specific hydrolysis rate f(T) [g Xs/(g Xbh COD day)]";
  Real K_x "Half-saturation (hydrolysis) f(T) [g Xs/(g Xbh COD)]";
  parameter Real mu_h_T=4.0
    "Maximum heterotrophic growth rate at T=15 deg C [day^-1]";
  parameter Real b_h_T=0.28 "Heterotrophic decay rate at T=15 deg C [day^-1]";
  parameter Real mu_a_T=0.5
    "Maximum autotrophic growth rate at T=15 deg C[day^-1]";
  parameter Real b_a_T=0.1 "Autotrophic decay rate at T=15 deg C [day^-1]";
  parameter Real k_a_T=0.06
    "Ammonification rate at T=15 deg C [m3/(g COD day)]";
  parameter Real k_h_T=1.75
    "Maximum specific hydrolysis rate at T=15 deg C [g Xs/(g Xbh COD day)]";
  parameter Real K_x_T=0.0175
.
.
.
.
 
  // Outputs
 
  Out.Q + In.Q = 0;
  Out.Si = Si;
  Out.Ss = Ss;
  Out.Xi = Xi;
  Out.Xs = Xs;
  Out.Xbh = Xbh;
  Out.Xba = Xba;
  Out.Xp = Xp;
  Out.So = So;
  Out.Sno = Sno;
  Out.Snh = Snh;
  Out.Snd = Snd;
  Out.Xnd = Xnd;
  Out.Salk = Salk;
 
  MeasurePort.Si = Si;
  MeasurePort.Ss = Ss;
  MeasurePort.Xi = Xi;
  MeasurePort.Xs = Xs;
  MeasurePort.Xbh = Xbh;
  MeasurePort.Xba = Xba;
  MeasurePort.Xp = Xp;
  MeasurePort.So = So;
  MeasurePort.Sno = Sno;
  MeasurePort.Snh = Snh;
  MeasurePort.Snd = Snd;
  MeasurePort.Xnd = Xnd;
  MeasurePort.Salk = Salk;
 
end ASM1base;



and I check it. It will say :
Syntax    14:30:47        32:3-33:2    No viable alternative near token: Base class of WWTP modelling by ASM1

It's because you have dots in your model name, i.e. WasteWater.ASM1.Interfaces.ASM1base, which is not allowed in Modelica. Either rename the model to something without dots, or enclose the name in single quotes. See the definition of IDENT in appendix B of the Modelica specification for more details on valid characters in model names.

There is indeed a syntax error in your code.

You are supposed to write:

within WasteWater.ASM1.Interfaces;

partial model ASM1base

  // ...

Thanks for the Help!

At least that Problem ist gone. More to come!